My first attempt at a mob farm seems to have been too close to the ground. I wasn't getting very many mobs. When I killed mobs at ground level, more mobs would spawn in the mob farm. How tall should I build the next one so that it isn't affected by mobs on ground level?
For an exp farm your mobs will have to fall 24 blocks to be killable on one hit. That way you get the xp and the special drops.
For mobs to spawn very efficiently you want to be 128 blocks away from any other potential spawning points for them (blocks with block light lower than 8), so you should be 128 blocks above the highest block in an 128 x 128 area around your spawner.