I like a break wire personally. Need to use 16-22 gauge wire so they can break it. Some guys use stronger wire or heavy braid and a retention pin that slips off the fencing under tension.
Run the wire around the trap through 1" steel rings tied to the fencing at least 2 ft off the ground.
At the back of the cage make the wire cross a section of the cage and tie the free end to a post 12" off the ground.
If the door is a heavy vertical drop door that is all you need. If it is a swing door style or is a light drop door, you will need to put a stop on the door to prevent it from moving further open when the wire is contacted by the pig.
You need to bait outside the trap sparingly but make sure to place a lead line of feed going into the trap out to and crossing a well used game trail.
Do not put bait directly in the door as hogs rooting there prior to triggering the trap can build up dirt preventing the door from closing properly. Do not get bait behind a swing door where they may prematurely push it shut.
Once inside the pin make two lines around the outside edge about 1 ft from the fencing without actually going all the way to the trigger. Place a small fruit flavored salt block in middle of trap (the lil ones run right to it). Between the fencing and trigger wire and underneath it, place a small deep pile of bait. I usually pour a little extra woured corn juice into the ground here also.
steel rings: Image: http://www.antonline.com/images/033923010627.jpg
retention pins: Image: http://www.heating-parts.co.uk/media/STOCKIMG/00091800.JPG