Usually if the crab is th size of a palm then i cut it in half then wrap some bait elastic around it, then usually use a two hook rig with the leading hook protruding through the legs out the bottom well exposed and then put the other through the top half near the mouth end and then wrap it up in bait elastic.
If the crabs are small ie smaller than the width of thumb lenght i leave the back shell on but give a couple of good wacks with the knife blade (to let the juices ooze but help keep the crab intact and prevent the lice cleaning it out) then put a hook through the legs and one through the mouth protruding out the belly with keeping hooks well exposed so to maximise hookup when they try the run and drop, hopefully a hook catches them, i then wrap it up in bait elastic (black magic bait buddy is excellent), with wrapping a few final wraps around a leg to keep the elastic in place and prevent it from coming off and wrapping around everything else. This is how iv done it in the past and it has proven very sucessful.
A fish i caught during a comp was taken on a half crab that was the size of a large hand and was amazed that the bait was taken no worries.
Hope this helps others.