I am trying to come up with trap where a room fills up with falling sand after they trigger the trap.  I would welcome and comments or suggestions to help me pull this off.  

Right now I am playing with the idea that there is a pressure plate at the top of the wall at the far end of the room.  They can shoot from a distance or try to press it as the sand rises. I realize difficult terrain would be in affect here.  That would give them about 3 rounds of action to get to the other side of the room.  I am also considering allowing to be a couple loose stone tiles that players could use to clog holes of spilling sand.  Anyways, I am not sold on any of this.  Just brainstorming.  I guess I am trying to also figure out a good difficult rating to set and if that would change with each round of rising sand.  

Anyways, any suggestions would be greatly appreciated.  

Pressure plate is fine and simple enough, mechanically.  You might want to spice that up by having multiple plates that need to be hit in order or that need to be pressed at the same time so the party needs to work together.

My general rule is to always have some sort of escape ready at the top of the room - you don't really want a possible TPK from a fairly mundane trap.  Large vents that the sand is pouring through is the classic but using this as the exit leaves them in a worse position than before - less treasure, longer route or more tribulations.  I like to try and have one of the party on the outside of the room a la 'Temple of Doom' (shout out to Short Round) as it adds a different element of drama and they can be desperately looking for another mechanism - but this will depend on positioning.

Sand traps are also one of the few times I use real-time countdowns as well - I just love the synergy of the sand trap and an hourglass - it makes everything a little more exciting.  If you don't have one there are some online, like here.

It depends on what you are using the trap for. 

Traps can  work 3 ways 

1. As a quick challenge. The player triggers a trap they get to react and then depending on the result they receive a penalty.  Basically you  tell them a trap is triggered, you may give them an clue which may be  a sound, smell ect...  and then ask them what they do. If it makes sense they roll the appropriate check. 
2. As dynamic terrain for combat. For example you may have mines that players could step on or have a room that slowly fills with sand changing the terrain 
3. As a penalty for a puzzle. After a certain amount of time or failed attempts they receive punishment. The players may have to decipher a code  while a room floods or something similar. 

Maybe start with the room partly filled with sand. Then while crossing it starts the drain into the floor like an hourglass. You can still keep your panel at the other end to stop the drain.

Difficult terrain with force movement towards the hole.

Where does the drain go? A deeper level in the dungeon, or into the stomach of a giant antlion, or ...