Minecraft wiki entry for Defeating_pyramids, associated text: Build a staircase spiraling downward along the walls.
Dig out the blue terracotta and use a water bucket to get down.
Dig vertically below your feet to descend, digging a vertical channel down one wall. After ransacking the room, step back into the vertical channel, and, using the blocks you collected on the way down, jump into the air while placing one of the blocks below you. In this way you can ascend back up the wall.
Dig a hole in one of the corners, drop in sand blocks until it gets filled, then use a shovel to dig out the sand below you. Gravel works, too. You can then use the same sand to climb out by looking straight down, jumping, and placing the block beneath you.
Just jump straight down. This is fastest, and it won't kill you. You will lose about 12 health points from the fall. Don't use a water bucket because the water can push you onto the pressure plate.