here are a few comercial crabbing tips, these tips may be a little over-kill but it produces. last winter we had over 6500lbs of dungies in 350 pots (one haul), and we didnt rig a pot any different than explained below (sorry if this is long, i felt like typing)
good bait is more important than location, but if you can find a sand patch, put pots there
this is the most important of all my tips: cockles and clams are crab crack
fresh bait is gold. go dig up some cockles, smash em up, and then get some herring and or squid(Prefered), cut it up small. keep the bait types seperated in different jars (use cockles shells and all). make sure your bait jars/boxes dont have holes that the bait will drift out of when the current picks up
if you get squid get some illix or something really inky. sometimes a chewy bag helps a little
if you are using round commercial style pots (vs the fold-away square pots coated with plastic) make sure that there is a zinc anode in the pot, and that all the metal is grounded to the zinc. the electrolis between the different metals the pot is made from creates an electric charge that the crab dont like. however the easiest way you can take care of this is to go to the cooler, grab a beer, drink said beer, and then cram the can under the tunnel. problem solved.
make sure your tunnels are tight, otherwise your gates can stick open or closed when the current runs, and then the crab escape. usually you can fix that problem with a nail or a screwdriver. tighten up the mesh by twisting.
if you are using 2 tunnel pots, set them out with the tunnels running perpendicular to the direction of the current, otherwise the current can open gates and let crab out.
hope that helps