Stretching after you workout is more beneficial to prevent muscle soreness and injury. If you stretch before you workout, there is more potential to tear a muscle. This is due to the lack of blood flow at the muscle.
To get the most benefit out of stretching, make sure you hold the stretch at a point you feel a pull within the muscle. Stretching should be held between 15-60 seconds. Perform 2-3 repetitions of each stretch on both sides of your body. If a stretch is painful, you should decrease the range of motion of the stretch.