try these:
http://www.lowes.com/lowes/lkn?action=howTo&p=Build/boxjnt.html
http://www.leestyron.com/lynnjig.php
http://www.beesource.com/plans/boxjoint/index1.htm
the sky is the limit on the degree of complexity when designing a jig. But the principles are the same. Any jig you choose must keep that stock tight and vertical for a good cut across the dado. That is the only way to have a tight box joint each and every time.
BTW. That Lynn Jig looks good. I might have to look into building it myself. It appears possible to cut the joint on multiple pieces of stock. Very Cool!