There was a an ab workout on t-mag.com last week by Thibadeau(sp), I have switched to that as i it's almost crunch/sit-up free and I can't stand doing lots of crunches. The constant contraction of the abs during the exercises is key.
i do 2 sets of 60 weighted decline sit ups, 2 sets 150 crunches, setes of 50 leg lifts,3 sets of 100 of side crunches, then 3 sets 100 of standing crunches!! its worked great for me and it only takes 15 min.
I do 3x12 decline situps with a 45. Then 3x10 weighted crunches with the whole stack. Works great, train abs heavy just like you would any other muscle.
I usually work from the bottom up. I start with hanging leg raises or decline leg raises, move on to a combination of leg raises/crunches such as a knee up or v crunch, and end with regular crunches.