Zir Red I agree with. Mobility and correct warm ups prior to training is critical to prevent injury. Stretching after training can help with flexibility, but you need to properly stretch. For example you are stretching the hamstrings. You are sitting down and reaching for you toes with your feet facing the sky. You stretch for 15 to 60 seconds. Let go for no more than 5 seconds then stretch again for 4 sets. This is the most effective manner to stretch, The mechanism behind this is your muscle spindle (MS) prevents from stretching, which is why you don't feel a stretch until later. After about 7 seconds the Golgi Tendon Organ (GTO) overrides this mechanism of MS and allows you to fully get a stretch. This form of stretching is good for flexibility and injury prevention. The only down fall of this stretching is it consumes time.
To add to this, an understanding of anatomy and kinesiology may also help.
For example, when most people stretch the hamstrings, they're spines are flexed, which does nothing to increase hamstring lengthening. Keeping the spine flat will ensure that your stretch is at the hip and not the 20 some-odd intervertebral joints.
In addition, the hamstring has no action at the ankle. However, point the toes up (dorsi flexion) increases the sensation of the stretch. Why? Because it increases stretch on the sciatic nerve. Keep the ankles plantar flexed, you release tension on the sciatic, and get a greater stretch of the hamstrings.
Interesting stuff.
Br