Don't do reps, I do them timed. do loads of sets, loads of exercises. around 15 in a session.

I don't use a bar or anything, and keep my feet together and flat on the floor, so I work my abs as much as possible... if you use a bar or don't keep your feet flat you're using your legs and hips as well rather than targeting the abdominals.

Takes a few months to build to that though.