this ought to be a good workout: it made my roomate puke last year. it works the lower back too.
alternate back hyperextentions with weighted decline situps. the back hyperextensions and situps should be weighted so that you can only do about 25-30. Do 4 sets of each without stopping. Then do 8 sets of 10 on the decline situps, resting only as long as it takes your partner to do them (this is the part where my roomate threw up, i believe on the fifth or sixth set). finish off with some more hyperextentions, sets of 15 or so.
I do this twice a week. you could get away with doing abs more frequently, (i really should, ive just been lazy lately). you could probably do them 3-4 times a week.