This is a little late, but I figured I would post just in case anyone else comes in here to read this thread.
As to situps.
First thing to remember is that abdominal muscle tissue is just that. Muscle tissue. It requires the same amount of recovery time as does other muscle tissue. You should only work your abdominals 1~2 times a week. If you are working abs every day, you are overtraining, and shooting yourself in the foot.
Secondly, you should remember to "train like you fight." When you do your abdominal workout, at least part of it should be conducted as if you are doing a real PRT. Learn to work within the system to maximize your results. You can learn little tricks (within the rules of the PRT) to get better. For instance: When doing the PRT, I try to flop down as hard as possible (without causing my butt to leave the deck) to minimize eccentric contraction during the exercise.
Another thing to remember: There is nothing wrong with training heavy. Using weights or machines to increase resistance in ab exercises will not "make you look fat" or cause you to "bulk up." Muscle tissue is one of the slowest-growing tissues in the human body. If you are bulking up, it is because of what you are eating, not what you are lifting.
And finally, to give yourself an extra boost, you might want to try doing a cycle of creatine the month before a PRT. You have to preload for a week, taking 4.5 grams 4-6 times a day, then dropping down to one 4.5g serving a day. Creatine is a naturally occurring substance in meat and fish. The amount of prescribed creatine would be equivalent to eating 10 pounds of beef a day during a loading cycle.
And again, since creatine has very few if any calories of its own, it will not cause you to "bulk up."