I do an hour of Pilates twice a week, and outside of that I do one day of crunches, supermans, scissors, leg raises, etc. I also like to swim 2-3x a week, and between butterfly (my fav stroke) and flip turns, that gives me a great ab workout. Nothing weighted any more--my abs respond almost too well to that sort of training, and I don't want them getting any larger.
If I'm in a time period when I'm not doing Pilates classes (usually there are 2 weeks off between sessions here, and no classes during breaks), I like to do abs 3x/week and alternate workouts for my obliques with exercises targeting my rectus abdominus/lower back. I did that before I started taking Pilates classes, and it worked very well for me.