if you need to make it heavier, putt a weightplate on your chest and do crunches. It all depends what rep range you want to be doing. do you want to go now over 15 and you can do 20 crunches with ease, then put on some weight until you can only do 15, etc.
I find that crunches / leg lifts to a great job... I don't want HUGE abs, thats not the point of working out abs, you want them to look defined. I think that crunches make them stand out, and look more lean, rather than being massive.