If you're unsure about the proper form, consult a trainer to give your workout a check up. Many people underrate the importance of warming up and stretching. Before your workout, spend some time jogging in place or doing jumping jacks to warm up your muscles so that you can stretch them easily. Never bounce while stretching and never try to stretch without warming your muscles up. If you play a sport, learn the proper techniques required by that sport.
Improper technique is linked to recurrent overuse injuries.
If you are unsure, take lessons or consult a pro. Buy and use the proper safety equipment for your activity, a helmet for biking, for example. Make sure everything fits comfortably and properly. Your shoes should match your activity and should be worn only for that activity. When increasing your activity, do not add more than ten percent per week.