Org Minecraft Server

[Sigurd]

To increase the spawn concentration, it's best to eliminate as much of the surrounding terrain as possible from possible spawning spots. As such, they're best built in areas that have been completely lit above and below ground (difficult to accomplish), or out in the middle of the ocean

My enderman farm is built on this principle.
Make it ideal for spawning by isolating it away from the mainland. Make a tower high enough for them to die if they fall off it. Make sure they only can spawn on the pressureplates that will activate the pistons that push them off to the killing platform (endermen needs 3 height on any given opaque surface to spawn and they can't spawn on water or half blocks (non opaque) including glowstone. They can't teleport to places that has water on it.)

I bet you could make a similar farm out in the sea as TinCow suggests. Have rings of opaque blocks with either tripwire or pressure plates that will activate pistons that push anything spawning on the rings. Make multiple rings of this kind with the lowest one at the killing height of mobs. Make sure the island ground level is lighted enough to prevent any mobs spawning there during the night. You can seal the tower to make it a night and day farm.. and make sure nothing can spawn on the top by placing non opaque blocks on the roof (e.g stone slabs or water sources). Then watch the killing field on ground level as raining mobs fills it up with drops. Make sure the thing is not too high as mobs despawn 128 blocks away from the player.

Yeah... these kind of farms only produces when the player is present.