crunches are by far the best thing i've found. not just straight up and down but the side crunches too. the ones where you lay on your back, put your feet 90 degrees and together and then crunch to the side and try bringing your feet to your shoulders kinda motion. it's crazy and you do those and the front as well (and by crunches which are more effective than sit ups - you need to just come up part way not all the way up) you'll feel it.