As a DM, I'm having trouble with my "poker face" . The player characters seem to pick up pretty quick if I'm hiding something from them so some of the surprises I've cooked up get spoiled before they're supposed to be revealed. On the other hand, sometimes I can make an encounter enticing enough for them to explore/interact especially if I was hoping it would further the plot along. 

A example from last session: Players arrived in a temple and there is a massive 30+ foot tall statue in the center of the room with a smaller more life like size statue at its feet, the latter with a very life like expression on its face. The table erupted with shouts of "medusa!" and "basilisk!" They were not wrong. Some time after, they were to encounter a medusa, I just wasn't prepared for the to put the pieces together so quickly. 

Another example: I wanted the party to find a folding boat on a dead body, because it would make travel away from their current location so much easier, but I didn't want to totally telegraph that they needed to go into the room where it was. Personally, if I was playing, I would generally explore 99% of a dungeon map, especially if I was in as good shape as the party was at the time. Eventually, they got the hint that I wanted them to go in there, but it felt a little forced on my part. 

So what advise could you give in these type situations? 

Well, the two examples you gave I wonder if they are less about poker face, and more about something else.

In your first example the players found a life-like statue and made the connection to creatures that they know can petrify people. I don't think that's a matter of poker face-- That's a matter of your players having knowledge of mythology, D&D, and possibly Harry Potter. This is going to happen in any game-- But you know what? It's not always a bad thing. Players enjoy being right as much as they enjoy being surprised. If they're predicting your moves every time, then maybe you throw a curveball. Listen to them, and switch things up on occasion to keep it fresh. Nothing is set in stone (pun not intended) until it happens in the game-- But don't be afraid to let your players be right, either!

For your second one, I feel this is more a problem with you determining a need for the players to locate something that is at odds with the way the players chose to play in that situation. In general I would advise away from any DM creating a situation where the players need to do a specific thing to move forward, as you can never be certain they'll do the specific thing. With this example, I think you'd be just as easy to have the dead body with the item on it in your back pocket. Then, as they explore, choose to drop it in at some point. I wouldn't worry about them needing to go to a specific room-- Simply have that important piece of information or important item appear where they did explore. You won't need to nudge them, and you'll get them what you want to get them.

As far as general pokerfacing: It's just a matter of practice. Come up with go-to phrases that you find help you deflect. "You think so?" Is one of my go-tos when the players posture a question at me. Or look down at your dice and roll them if that's what helps you stay stone-faced. You'll find what works best for you by experimenting.

Awesome! Thanks for advice. I'll definitely try "you think so" ! 

A DM face is something that happens over time, but it's not just body language that helps in these situations.

First you have to understand that there are preconceived notions that players are going to go into the game with. The idea that there are Basilisks and Medusa in the world is not far fetched, so that is a logical jump. You also have the idea that the players are ignorant, what may seem obvious to you is going to be missed by them. What we (DMs) consider as heavy handed hints may simply be missed by the players. Where as evocative descriptions will catch their imagination and make them try to guess what's coming next.

When it comes to body language, the best advice I can give is to try to stay relaxed. You know what's going to happen, you've got the dominoes set up, and you're just waiting for the players to start the show...relax, it's going to happen or it's not. The more you can just go with the flow, the less you're going to give away when it comes to those shock/surprise moments. I spent 3 years setting up a betrayal arc on my players and I tell you it was impossible at times to just keep a straight face when they'd choose certain actions and make their different decisions.

The other part to the situation is the choice of words you choose. The more focus you put on something, the more the players are going to assume that it's vital to the scene. Try to give the same amount of narrative flare to the mundane as you do the special, it'll help keep them guessing and it'll make your job easier all around. It's also important to use the same approach to certain answers as well, this way they get the same type of response for the regular chest and the magically enchanted end of the dungeon chest, or the mimic. Verbal cues are harder to contain as we get excited about our creations and it shows in our language and vocal changes. 

Example:

"You enter the room and there are two statues. The immediate sight is a giant statue standing at least 30ft tall depicting a great Human warrior in scale mail holding a battle axe as though he's getting ready for combat. The second is a statue that is severely dwarfed in size by comparison. This is a statue of an Elf, the bow is drawn with an arrow in it, wearing leather armor. The smaller statue seems less weathered than the giant one, but it's made of the same material. The rest of the room seems to be well kept, the sconces are lit and seem fresh, the wooden door on the far side of the room, behind the giant statue, is closed.
Or
"You enter the room and see a giant statue in the center, it depicts a great warrior. At it's feet you see a statue of an Elf, bow is drawn as if in mid fight, the leather armor, quiver and even the look on it's face are immaculate in detail. The giant statue seems to be worn with age in comparison. On the far side of the room you see a closed wooden door."

Though the second is a shorter description, you can see that the detail is focused on the smaller statue, making the giant one just a set piece. Where as the first description puts detail into the room, both statues, and some fluff to make the whole room appear in the mind's eye. There's just a hint in the language that the smaller statue is different, but it could be attributed to a few things.

The last thing I can offer is to create a few red herrings, give them evocative descriptions about a thing, place, or person and have it lead them to something mundane. Don't make it a contest for the players to guess what's true and what's not, instead just have a few moments where you mess with their expectations. In the case of the Medusa/Basilisk situation, let them have it, they guessed right. Later on down the road give them an almost identical situation, a garden with very lifelike statues. Then as they walk through this garden, weapons drawn, spells at the ready, they come across a Wood Elf who is simply a master sculptor.

Hope some of this helps, and remember: It doesn't matter if you have a poker face if you're running an interesting story!

Gary Gygax used to hide behind the open drawers of a file cabinet in his basement games. His players would hear his voice and descriptions, but never see his face in those sessions. I don't share this to suggest you run out and buy a filing cabinet... but you can get creative or at least take solace in the fact that Gary hid his face :) 

There are many stories I've heard about DMs back in the day going so far as to have full shields to hide themselves from their players, robes that covered their faces, DMing from other rooms, things of that nature. Some did it to make it so that the players felt that they were the only thing important to the game, some did it because it created an aura of mystique, some did it because they felt the DM wasn't a player, and I'm sure some did it because they didn't have poker faces as well. 

Thanks so much for the thoughtful reply. I'm going to try and work on my descriptions like you suggested. 

Some of what you said has me in a bit of an existential crisis. I've been mulling over the following questions for a bit (How do I convey to the PCs that sometimes combat is just combat? What can I do to encourage the PCs to explore more? ) , but your comment has me thinking the answer may just be as simple as I'm not doing that great of a job as a DM descriptively and narratively guiding them.   

I would like to reiterate DMThac0's comment about dialogue. Be consistent. If you would normally keep your descriptions brief with basic detail, suddenly waxing poetic about a statue, or a particular wall, or a fallen monster, is going to get your players spider senses tingling. For myself, I tend to keep things brief, but colourful. And if characters want to investigate, then I give them more detail. So in Thac0's example....

As you enter the room, you notice two statues, shadows flickering wildly from the sconces burning brightly on the walls. The first statue, to your right, is a giant warrior, wielding an axe, while a smaller statue to your left depicts an Elf with a bow. On the far side of the room beyond the statues is a closed wooden door. (Nature check). It looks like Teak, but hard to tell from where you stand. (I also love throwing in stupid bits of info like that. Take that last bit with a grain of salt).

- We take a closer look at the statues.....

(you could make them roll, or not, depending on how frugal you want to be with your info.)

You inspect the statues carefully, awed by the craftsmanship. In all your travels, you have never seen a statue so lifelike, almost as if it would draw breath any second. The stone is smooth and flawless, and you notice that it surely must have been made from a single piece of stone, as there are no seams, no joins that you can see. Such beautiful statues, they would be worth a good deal of money to an art collector. It seems strange that they would be places so haphazardly, all the way down here.....

The most common thing I hear from DMs, when they refer to what they'd like to improve, is narration. It will always need to be refined, no matter how long you DM for. I've got 30+ years behind me, I've written stories for video games, written poetry, I'm working on a novel, and I've DM'd for most of those 30 some years....and I still think my narration could use improvement.

Don't sell yourself short, however, if your players are enjoying the game then you've got a wonderful foundation to work from. Just take the time to slow down and visualize what you want to describe to your players. Give them no less than 3 of the 5 senses in your descriptions to help them visualize the scene. You'll improve the more you do it, and the more you relax and enjoy the time behind the screen.

edit: Combat is never "just combat", it's a matter of opposing goals, and it's important. Though the goal may seem to be as simple as "I want to kill you" the piece that's missing is Why. The killing is the means to the goal, the goal is, generally, something much different. Is it for money, fame, or sport? Is it to protect a home, a person, an important object? Is it an order that must be followed, or is it a personal vendetta? (Yes, sometimes it's simply because "I'm hungry").

Once you figure out the goal then combat becomes a story in itself, it changes strategies, and it creates a reason for combat to shift as it goes on. Will your opponent(s) fight to the death or will the flee when it gets ugly? The goal is where you figure that out. Are the opponents going to know about the party members? The goal may give you reason to say yes or no. It may also give the enemie(s) a reason to stop combat and bargain or, if taken hostage, give them reason to divulge information or die to keep the information.

Easy and Cheap Fix:

The easiest way to hide things from your players is to mess with their perception of how you handle things. If you act like you're hiding things when you're not hiding things, it becomes more difficult for them to tell when you are genuinely trying to bluff or conceal something. I was smirking like an idiot in my last session, and my players were convinced that I was up to something. In reality, I just had this vine stuck in my head and couldn't stop myself from laughing at it, then I moved onto laughing at the fact that they were trying to work out something that wasn't there to be worked out.  

You've also just got to be aware of the fact that the players perceive the game differently to you, and each of them has a different perception from each other. They're going to know the tropes, and sometimes something that you think is really subtle and clever will be really obvious to them. Other times, it'll be the exact reverse. There's not really any controlling that, you've just got to go with the flow and be prepared to adapt to your players needs.

Fake them out. They find a chest that looks to be made to seem like it it has teeth? Actually just a cool design. Another room the find has a mimic in it.

If they start trying to guess, just respond with "potentially", even if the guess is wrong. I was DMing a one shot where they were at a banquet, and the Mayor said, "The whole town is here! Except for Harold. Nobody likes Harold." One of my players instantly assumed he was going to be the enemy. He was correct. I responded with "Potentially" and it made them think.

It's also fun for them to say, "Ha, I told you that there was going to be a Medusa here!" It's exciting when you are right. Just keep that in mind.

Quote from Rumpelfarts >>

Thanks so much for the thoughtful reply. I'm going to try and work on my descriptions like you suggested. 

Some of what you said has me in a bit of an existential crisis. I've been mulling over the following questions for a bit (How do I convey to the PCs that sometimes combat is just combat? What can I do to encourage the PCs to explore more? ) , but your comment has me thinking the answer may just be as simple as I'm not doing that great of a job as a DM descriptively and narratively guiding them.   

 If you want them to search out the majority of the dungeon, place several chests in different places including some that are hidden. Then, have a dead explorer with a note or journal suggesting that great treasure exists in the dungeon. If there are some really sweet finds in the hidden chests, it will condition them to search for things or you'll discover that they aren't that interested in that aspect of the game. 

I am terrible at keeping a stone face lol, so I just go with it and my players make their guesses.  Sometimes I purposefully lead them down the garden path by letting them totally run with what they "think" is going to happen 

Like if they were meant to face a medusas and they guessed it, I'd let them run with it when it gets to the battle, instead of a medusas, they find a half eaten medusas and must face a creature immune to petrification - who has just killed and devoured the original monster they were meant to face.

But if my players are all like looks like a medusas but you know he's going to spring a surprise on us, they'll face the medussa.

So yep - the surprise is there was no surprise lol.

Keeps my players guessing and on their toes.

Then sometimes, I just let them be right because players love being right and working out your plot.

Sometimes if my players are having a bad day, I will straight up tell them (via an NPC) "there's a medussa in there, you want to go in well equipped and prepared for a hard fight and watch out for her gaze"