What's up man,
I'm no expert so the following words are only from my own experience.
What has helped at least me are overhead squats w/ bb, and hip hurdle mobility drills.
Hip hurdle mobility drills i got from Coach Davies' book, check out his site or maybe even T-Nation, might have them somewhere. If you don't have hurdles, Cressey's (I believe) article "10 uses for a Smith machine" is probably as good a read as any.
Overhead squats, Dan John has written a lot about them, search T-Nation and/or his site. Start w/ a broomstick, really try to get to the bottom with good form. Maybe u could try to do some contract/relax when at the bottom position of a OHSQ. Sometimes, if u try a barbell instead of a broomstick, or add weight in general, u might notice that its easier to get to the bottom cause the weight kind of pushes you down. When using weight though, tempting to not go all the way to the bottom.
Also, a good morning hamstring stretch, PNF-style could help u loosen up ur hamstrings. Basically u reach ur butt back, chin forward, balance w/ hands, to a point where u feel a stretch/tension in ur hams. Then u squeeze/create tension in ur whole body, and release the tension, which should cause the stretch to deepen.
Hope this helps.