The great thing about the various kinds of crab traps is that they work well with little effort and expense. You can purchase crab traps at tackle shops and some hardware stores for 8 to 12 dollars, depending on the size and type. Crab net rings and the different shaped collapsible traps all operate in this way. When the crabber pulls on the cord, the sides are pulled up and if the crabs are eating the bait, they will be trapped inside.
Chicken necks are a good bait for crab traps.
Necks are inexpensive and easy to find in the meat department of most grocery stores. You may use fish parts for bait, but the bone in a chicken neck allows for secure attachment to the trap, and it holds up longer against the ripping and tearing as the crab feeds. Tie the bait to middle of the bottom part of the trap with wire or heavy cord. Lower the trap in the water with enough cord or rope so that it sits on the bottom. Then tie the other end of the cord to the pier.