Since I've been out of the gym for about almost 3 weeks now and unable to do cardio, even eating healthy I've lost my 6 pack. I'm down to about a 3 pack LOL It sucks!
Anyways, in regards the question I typically would train them EOD or every three days for about 5-10 minutes with little break between sets (10-20 seconds) I would vary exersices (bicycles, leg lifts, oblique crunched, etc.), but as it was said before those basic strength movements like squats and deads will work them just as well. Just make sure you're lower back is up to par like dreww stated (deads, superman extentions).