The rectus abdominus is all one muscle that runs from your ribcage to your pubic bone, so technically, if you're working that muscle out, then you're working the whole thing!

I'd suggest cutting back to 2 or 3 days a week and perform exercises that involve using external resistance other than bodyweight- like weighted sit ups and leg lifts or (if you go to a gym), ab machines. You can also use an elastic band attached to something for resistance.
Just like any other muscle, you need to increase the difficulty if it starts getting too easy, and, try to keep the reps under 15 (ok, maybe 20 if you HAVE to, but you're better off adding more weight).