Ahhhhh...
When I was a collegiate athlete this was always a big thing. They wanted to make sure we hit BOTH and hit them hard.
For JUST abs, here are some suggestions:
1) Use the bar thing (I don't know its proper name, but you put your head and arms in it and it forces you to roll rather than lift)
2) Never lock your feet to something to do crunchies. If your feet are under something your flexors will be a big factor.
3) When doing all types of abs, focus on curling up rather than lifting your shoulders. To hit abs your torso must be bent. Keeping it strainght=hip flexor workout.
4) Keep the small of your back on the ground when you do crunchies.
And here is a general tip: Abs are just a muscle group like hammys or anything else. While doing 100,000,000 crunchies will help your endurance, it won't make them particularly strong. Would you do 1000 curls if you wanted max strength from your Biceps?
Use weight and lower reps from time to time (at least). 4-5 sets of 10-15 twice a week will really blast your abs. These should be to failure or there about, just like you would do for other muscles. Doing these sets will also help you do more sit-ups, like in the hundreds.
This is akin to runners lifting with their legs. It makes them stronger.
I could go on about this forever, and explain more nuances, but I will spare you.