I haven't done any ab work for months cos I'm a lazy c*nt. When I was doing them I started off with bodyweight crunches and lying sidebends. I then progressed to weighted crunches on a Swiss ball, holding 20kg plate behind my head. I also did a few sessions of dumbbell sidebends. These can be tricky to hit the muscle properly as correct form is important.
For some strange reason (probably poor form) I found the weighted crunches a fair bit easier than bodyweight. I'd do 17-18 with strict form at bodyweight and stop with my abs burning with pain. I'd do 30 reps with the 20kg plate and even though it worked abs reasonably well I'd mainly stop the set due to boredom. Something to do with the bloody swiss ball is my guess, cos I did the bw version flat on the ground.
As I said I don't do em at all now, not really a high priority, but I plan to extract the digit at some point and get back to it.