What works for me varies but what's working for me right now?
Decline crunches with 100lb dumbell held on my upper chest, 3 sets of 5 to 8, when I get 3 sets of 8 consistantly I'll up the weight. Twice a week.
I get a lot of twisting and such in during jiu jutsu and as part of our warm up we do a couple minutes of leg lifts/splits, etc. and this is 2 or 3 nights a week.
For me, heavy, moderate tempo compound movements have always worked best, especially the big 3, flat bench, squat, dead.