To avoid injuries, you should always warm up before stretching. A good warm-up consists of three parts:
Unless you've just woken up or have been sitting for a long time, you don’t need to do the pre-joint warm-up.
When I talk about a joint warm-up, I don’t mean traditional joint circling (e.g., pressing your toes down and circling your ankle on the floor). That kind of circling is okay, but you shouldn’t apply pressure or force the joint through its full range of motion. The goal of a joint warm-up is to lubricate the joints, not to twist or compress them. If you squeeze or forcefully rotate your joints, you’re more likely to wear them out than warm them up.
In the muscular warm-up, your muscles and tendons get prepared for more intensive work. I recommend doing a joint warm-up for 3 to 5 minutes, followed by 10 to 15 minutes of light cardio before you begin stretching.
Find more information about stretching and related exercises at Stretching Main.
Here’s an example of a leg warm-up I use before intense stretching sessions. Feel free to substitute with similar movements.