One more thing to add . . . You said you do 300 crunches each time, so I just wanted to point out that if you are doing that many, you probably aren't doing them quite right. Your abs are like any other muscle -- you wouldn't do 300 bicep curls, right? I recommend the book Fitness for Dummies, which has some info about crunches. Basically, you need to make sure you are actually using your abs to lift your shoulders off the floor, not just pulling your head with your hands (which can strain your neck). Also, you probably want to do them a little slower, and hold in the crunch position for a few seconds before you lower back down.
Also, if doing them on the floor is too easy for you, there are some other options: first, you can do them on an inclined bench, so that your feet are higher than your head. This really increases the resistance and is my favorite. Also, you can hold a weight in your hands, above yourself, while you crunch, which also adds resistance (but I would worry about dropping the weight on my head!). Anyway, I love the incline. I could do at least 50 crunches on the floor, but I can only do 20 or so on the incline which is just right.
BTW, I'm no expert, but you knew that already.

Good luck!