Make warm-ups and cool-downs a permanent part of your exercise program. Warming up and cooling down are important parts of an exercise program. They help to keep your muscles flexible and lower your risk of hurting yourself while you exercise. Warm-ups and cool-downs will also help you to slowly raise your heart rate before exercising and gently slow it down afterwards. That way, you will not suddenly stop and start your exercise, which is not good for your body. Before you exercise, warm up by jogging or walking in place quickly until you "break a sweat". Then, stretch for five to seven minutes. After you exercise, cool down by doing the same thing--jog or walk for a few minutes, then stretch for five to seven minutes.

These websites list many stretches that are good to use for warming up and cooling down:

Stretching Exercises for Women
Warm-up Stretches Prior to Exercise

Hold stretches for at least 15 seconds. Do not bounce while you stretch, just hold the position. Remember that when you stretch, you should be able to feel a stretch, but it should not hurt you. It is best to stretch your muscles once you have already warmed them up. Stretching your muscles before physical activity (when they are "cold") can cause injury.