One of the worse things you can do is place your hands behind your head or neck. It ads strain to your neck and back. Cross your arms across your chest to allow you to still do sit-ups and crunches. Plus if your used to doing it this way you can hold a weight on your chest to add more resistance to your work out.
Like it was said before the best type of work out is one where you use a varied workout to isolate and work on different muscles. This will give you a more refined shape if that is what your wanting.
For you ladies who just want to look pleasing and not gain muscle mass use a low impact work out to raise your heartbeat and achieve a nice calorie burning exercise without worrying about being muscle bound. Crunches, pushups, leg lifts and the even an nice 3 to 4 mile walk will keep you feeling good and keep that nice figure you want.
The biggest thing is to make sure you stretch weel before and after exercising to minimize injury. Ladies being more flexible is a plus to your man.