You don't have to be doing endless sit ups and crunches to get that nice flat and toned stomach. Here is a short routine that will strengthen the core muscles just as well.
Plank position on your hands, drive the hips up raising one leg in the air, then as you lower the hips bring the knee through the hands and flatten out the body. Repeat 4 times.
Then slightly alter the movement by bringing the knee to the outside of the elbow (on the inward movement) to engage the oblique muscles. Repeat 4 times.
Then switch legs...
As you become stronger build up the reps and then try and go for two sets.
Remember: Keep the core muscles engaged throughout by thinking about squeezing the belly button towards the back.