get a masons jar, 20oz pepsi bottle cut the top 1/3 off the bottle
invert it into the mason jar and put the lid on( with out the cover)
put a chunk of frozen food in jar
submerge the jar in bottom on it's side so the crab can crawl in after the food
i caught mine at night without a trap.. i saw him once and i couldnt get him about a month later i saw him again couldnt get him the next night say him and just scouped him out.. so keep ur eye out at knioght
Similar to what NYPDFrogman suggested, I put a drinking glass in the tank leaned it up against the rock near a usual hang out for the crab and put some food in the bottom. The crab goes in to get the food, but can't climb out of the glass due to the smooth sides. It took two nights, the first night I caught a bunch of hermit crabs, the second night.. goodbye crab.
The tilted glass (16oz beer glass) method has also worked well for me, I used half of a raw shrimp and watched the sob climb in and boy-oh-boy he did not like getting stuck in the glass, off the the lfs and he was an appetizer for a nice sized trigger. He went form predator to prey.
Get a plastic box (tupperware works if clean and no soap residue), and put food in the middle. Than place a ramp up to the box, (the lid from the tupperware would work). Position the trap near the crab, it can get in, but after it falls in for the food it will have trouble getting out.
I think this was mentioned, but I am trying to boost my post count.