Similar to this...except I walk on an incline for 5-10 min, just enough to get a sweat going, then do 5-10 min of normal abdominal exercises. Great way to warmup and get the blood flowing. Also, I will do one workout using weights per week. Usually something like weighted leg raises drop sets, cable crunches drop sets, side twists, etc. Enough to where when I'm finishing them off and squeeze extra hard, they start to cramp and catch. Hard feeling to describe, but you know it when you feel it.