Try slow stretches and go through the motions of your sport or activity before and after you workout session.
Stop the activity and seek medical advice if you feel discomfort or pain, become extremely breathless or develop an irregular heartbeat.
You can lose around one & a half liters of fluid for every hour of exercise; so drink water before, during and after a session.
Make sure your shoes and equipment fit properly and are right for the activity. Check you equipment regularly for safety.
Take it easier in hot weather and wear clothing and sunscreen to protect you from the elements.
Training too hard or too fast can cause injury. Have at least one recovery day each week to rest.