I read this great article this morning on the importance of not overstretching. It came to me through the magic of Facebook by way of two great massage therapists in Midcoast Maine, Kristen Burkholder and Katia Ancona. Remember that your tendons and ligaments have very little blood flow, and so they take a very long time to heal if damaged.
My massage instructor, Donna Kraft Smith, who was also a yoga teacher, used to say when we were doing our daily yoga practice "know the difference between the edge and the wall". The edge is where you feel resistance, this is where the work should be done. The wall is the place where you feel you can go no further, listen and respect that boundary to avoid injury.