I have recurring hamstring strains!?

I have not torn my hamstrings, the worse i've done is pulled them. Maybe this can a help a bit. What i do to loosen my hamstrings before I work out (I'm a gymnast by the way) I do the regular stretching and what not, but after I lay on a medicine ball and roll my hamstrings around, this actually helps loosening them and if you have any knots it helps too. I have not had a pulled hammy since then. Hope it helps. Most importantly though keep doing your physical therapy.