I agree with Jennifer... we don't do full sit ups in any of our gym classes - which are taught by professional fitness instructors, so I'm assuming they know a thing or two... (I have heard they are not good for your back anyway?)
The smaller situps that Jennifer described are called crunches I think?
You can also work the sides (obliques) by crunching and aiming one shoulder to the opposing knee - like a diagonal crunch.
You can also do leg extensions, which will work the muscles of the lower abs.
Lie flat on your back with your legs up, and knees at a 90degree angle. Then slowly extend one leg out horizontally/straight, and then back in again to 90degrees, then alternate to the other side and so on. Make sure that your lower back stays flat on the floor whilst doing this. When you get stronger at it you can also extend both legs at the same time.
HTH