In my games, I sometimes use poster maps given in pre-made campaigns but they give away the rest of the dungeon. How should I work around this? What is the best alternative? Please help!

Fold the map up or tape plain sheets of paper around the part you don't want the PCs to see.  

There aren't great options, but you can use sheets of paper to cover the areas you don't want them to see.

I like fabric to cover my dungeons. It’s easier to fold it and bend it around as the party enters each room. 

Last night I used sheets of paper towel in a pinch to cover what characters couldn't see. Quick, cheap, and good enough for me.

I have used sticky notes, black pieces of cardboard, cloth or felt, cotton (to simulate fog or clouds), or printed out roofs as fog of war on poster maps.

Honestly, I simply don't. 

Suspension of disbelief combined with remembering that player knowledge does not equate to character knowledge is an important thing. Besides, just because players can see the shape of a room or upcoming hallway, does not mean they know what's populating said room or hallway. It can even lead to a different type of fun. There are players I know who've experienced no end of hilarity out of the fact that they as players know that there was loads of stuff left to explore beyond the areas they managed to access (due to a failed Lockpicking check).

Just being able to see a room decked out for a feast, doesn't reveal that there are a hoard of goblins sleeping under the table. Likewise, seeing a stone chamber of coffins isn't the same as the GM describing the musty smell, the slight clicks from a bat nesting in the cracks of the stone roof, the cold, but oddly humid temperature. Nor does it give away the creature camouflaged against the stone roof, just waiting to ambush any prey who wander beneath it.

Therein I feel lays the key. Knowing some stuff, doesn't mean knowing all of it. I've run the Mansion in Phandelver a dozen or so times each populating it in different ways, each time with different puzzles and solutions. Players might know the layout, but often that's all they know. I've said it before on these forums, don't get too worked up about the idea of spoiling something. Life is experiential, we as humans like to experience things and even if we have an idea of what to expect. We see the rollercoasters, even ride them, but still want to go on them a second time. 

Short of having a maze or labyrinth, there's really no need to worry about concealing the shape and layout of the map. If the players are focusing too much attention on the map in front of them - you've likely got bigger problems than potential spoilers.

Quote from martintheactor >>

Honestly, I simply don't. 

Suspension of disbelief combined with remembering that player knowledge does not equate to character knowledge is an important thing. Besides, just because players can see the shape of a room or upcoming hallway, does not mean they know what's populating said room or hallway. It can even lead to a different type of fun. There are players I know who've experienced no end of hilarity out of the fact that they as players know that there was loads of stuff left to explore beyond the areas they managed to access (due to a failed Lockpicking check).

Just being able to see a room decked out for a feast, doesn't reveal that there are a hoard of goblins sleeping under the table. Likewise, seeing a stone chamber of coffins isn't the same as the GM describing the musty smell, the slight clicks from a bat nesting in the cracks of the stone roof, the cold, but oddly humid temperature. Nor does it give away the creature camouflaged against the stone roof, just waiting to ambush any prey who wander beneath it.

Therein I feel lays the key. Knowing some stuff, doesn't mean knowing all of it. I've run the Mansion in Phandelver a dozen or so times each populating it in different ways, each time with different puzzles and solutions. Players might know the layout, but often that's all they know. I've said it before on these forums, don't get too worked up about the idea of spoiling something. Life is experiential, we as humans like to experience things and even if we have an idea of what to expect. We see the rollercoasters, even ride them, but still want to go on them a second time. 

Short of having a maze or labyrinth, there's really no need to worry about concealing the shape and layout of the map. If the players are focusing too much attention on the map in front of them - you've likely got bigger problems than potential spoilers. 

I feel this approach overlooks one key aspect of the game—the joy of discovery and its role in the exploration pillar of the game. The reason a lot of DMs use a fog-of-war approach to dungeons is regardless of how well players can suspend disbelief and roleplay, it's still fun to explore a dungeon and discover it's layout. Finding secret rooms not because you could see there's an adjacent room with no doors, but because you were examining the room you're in and found the secret lever is fun. This also happens to highlight how DMs shoot themselves in the foot when it comes to enabling all three pillars of the game. The DMGs both provide suggestions on concealing parts of the map—draw it as your players go, cover pieces with paper, or cut the map into rooms and lay them down as you play—because that's part of exploration.

As for actually responding to the OPs query—I will either cut up black poster-paper and use that to cover parts of the map, or make a copy of the map and cut that copy up into individual rooms. Or if I wanna be super extra and I have the time, make the map in 3d and print it out with my 3d printer

Quote from Davyd >>

I feel this approach overlooks one key aspect of the game—the joy of discovery and its role in the exploration pillar of the game. The reason a lot of DMs use a fog-of-war approach to dungeons is regardless of how well players can suspend disbelief and roleplay, it's still fun to explore a dungeon and discover it's layout. Finding secret rooms not because you could see there's an adjacent room with no doors, but because you were examining the room you're in and found the secret lever is fun. This also happens to highlight how DMs shoot themselves in the foot when it comes to enabling all three pillars of the game. The DMGs both provide suggestions on concealing parts of the map—draw it as your players go, cover pieces with paper, or cut the map into rooms and lay them down as you play—because that's part of exploration.

As for actually responding to the OPs query—I will either cut up black poster-paper and use that to cover parts of the map, or make a copy of the map and cut that copy up into individual rooms. Or if I wanna be super extra and I have the time, make the map in 3d and print it out with my 3d printer

 
Can't say I actually disagree with this approach. My in person games and my online games run inherently differently. In person, I'm more likely to use either good old-fashioned grid paper approach or just out and out theatre of the mind. So, I don't personally tend to have the problem of a poster map I have to hide. If I am running a game with a map form a published module or some such, I've started using Owlbear to cast to a TV screen or computer monitor so that we can all see the same visuals. If I absolutely had to use a poster map though, my personal approach is to just minimise the extra set-up time necessary in concealing some of that map.

I can quite see how some folks enjoy the exploration aspect of the game, and if you're using simpler black and white outline maps again much of that exploration occurs within the description of the room which has just been entered. The purpose behind my comment was not to say anyone's wrong, but rather to offer a differing perspective from my own experience.