Here's a list of 5 common exercises that are actually bad for your health! I was surprised to find this one:
Sit-ups: Not only are sit-ups bad for your neck, but they're also one of the least-effective abdominal exercises you can do, according to a recent study at San Diego State University.
The bicycle sit-ups they describe are also very good, but are too hard for the person just starting out on an exercise routine. Build up the number of sit-ups you can do before you move onto those.