Aside from a few bodyweight-only exercises, I like to treat the abdominals as any other muscle group - with as heavy weights as possible. This is particularly true of the obliques, and a favoured movement is the one-handed rack pull.
This is essentially the same as a dumbbell side bend, only using a barbell. The length of the bar makes it that much harder to control - consequently there is a lot more grip and wrist work involved than the humble side bend. It's also much more fun.