Don't Do This in Your Warm-up:
Don't static stretch before you work out or play a sport unless you've been specifically instructed to do so by your therapist or doctor. Static stretching (holding a stretch for an extended period) doesn't prevent injuries and it fact may make you weaker and less powerful. Don't spend a lot of time doing aerobic work (running on a treadmill, riding a bike, etc). A few minutes (3-5 minutes) is okay but don't bother with an extended aerobic warm-up.