I find a very effective trap to be a very deep 1 by 1 pit with a hopper and chest(s) at the bottom. At the top, there should be a button that opens a piston leading into the pit. The top part of the tunnel should be disguised as a water elevator with a sign saying "This is a trap. Trust me." When they start down the elevator, they will reach the part with no water, fall down, die, and have their items moved into the chest(s) at the bottom.
This won't work for servers with the /back command, though. Luckily, almost no PvP servers have this command enabled.