Do combinations of different ab exercises. For example, without stopping, do:
50 x Crunches
10 x Hanging Knee Raises
50 x Side Bends (to each side)
50 x Crunches
Finish by holding stomach vacuum for however long you can.
Or you can do more or few exercises, do however many you want in a row, or just one exercise at a time. Do weighted ab exercises too. When doing crunches, make it a point to achieve a quality contraction. Give the muscles an extra squeeze at the peak of the contraction. Take it slow and keep it quality.
I think the best way to develop abs is to experiment with weighted and non-weighted ab exercises, in different arrangements, frequencies, intensities, ect.. Get instinctive and you will figure out what works best for you. Make sure to watch for overtraining, as it's easy to do. A good indicator of overtraining would be something such as your muscles fatigue way before you think they should, not being able to achieve high intensity, or not achieving a pump.
Do your stomach vacuums! I do vacuums after every ab exercise. I even do them when I'm just sitting around home, or wherever! I don't think it does much for developing the muscle, but it improves your ability to keep the muscles tensed. Hell, some people think it does improve definition. I don't think you'd have to worry about stomach vacuums leading to abdominal overtraining either.
I honestly think people pay too much attention to ab training. Even people who don't put forth a lot of effort might still have decent abs if their body fat is low enough to show them. But I'm not saying you shouldn't be performing quality training for the abs. Quality abdominal training results in quality abs, and low body fat reveals them.
Pretty simple, I think!
Or just type in ab workout on google, or any bodybuilding article database search and you're bound to find hundreds of ab workouts. There's probably more workouts out there for abs than any other muscle group. I wonder why? :laugh: