Minecraft wiki entry for Things_not_to_do, associated text: The simplest and quickest way is to dig a channel straight down the sidewall, collecting the sand blocks on the way down. To ascend again, jump while replacing the blocks underfoot.
Dig down in a staircase fashion, and simply walk up the stairs to ascend back up.
Use a bucket of water to fall down to the treasure room at a lower speed, making it easier to avoid the pressure plate. Ascending can be done by digging stairs or placing ladders.
Mining the pressure plate and/or TNT