I once read a thread by "needsize" where he suggested these amazing static holds. You lean back on a decline bench until you're at max. resistance (more or less the midway position if you performed a situp) and you just hold it for five seconds or so. That's one rep...and you shoot for five sets, etc.,
Anyways, those are all I've been doing for the past two months, maybe three times a week. Never been happier with my abs and worked so little on them.