Increase flexibility without stretching...
Here is my favourite hamstring 'stretch':
Lay on your back with straight legs. Now lift your straight right leg to the point of very mild tension. Roll up as if doing a crunch and place both hands on your knee (placing both hands on the shin or one on the shin and one on the knee works even better), Slowly push with straight arms against your knee but resist with your leg. Build up tension to a near maximum and hold this isometric contraction for up to 10 seconds. You can push as hard as you comfortably can. RELAX SLOWLY and place the leg back on the ground.
After a while when you are recovered lift the leg again, but this time with closed eyes. Stop when the perceived hamstring 'stretch' is comparable to the stretch in the starting position. You should have gained at least a couple of cm. Now pull your knee to the chest straighten the leg to a mild stretch and place both hands on the shin (if that is difficult hold your knee with one hand and place the other hand on the shin). Try to press the shin to the ground but again resist and build up isometric tension, additional press your left heel into the ground as if you were trying to lift your hip from the ground (but don't lift the hip). Hold the isometric tension for 10 seconds and RELAX SLOWLY and place your right leg back on the ground.
After a while try to lift it again with closed eyes to a point where you feel the tension is comparable to the start position. Again an improvement. From here on start a 'traditional' hamstring stretch. The process can be repeated several times.
This is excellent to know when you have a pulled or strained hamstring as you can loosen it up this way quite a bit without the need to stretch it. If you use it for this purpose stay with a relatively low intense isometric contraction (20--30% of max) and hold it for up to 30 seconds and repeat several times.
It is also a good way to finish a classical lying hamstring stretch. Depending on how I feel it works even better after a classical stretch. Unfortunately it doesn't work for the quadriceps, as the hamstrings tends to cramp when one contracts them while in a quadriceps stretch...