I'll second the HAM/HUG drills from Mtn Athlete.
Something else that has helped me tremendously since my injury has been sitting on a leg press machine (Gasp! I know, anathema!) with like 50 lbs on it, and doing 30 sec holds at the bottom position, and when I'm here I scoot the seat all the way forward so that my knees end up by my ears almost, and then 30 seconds with my legs stretched all the way out, for like 15-30 reps. The whole thing takes about 5 minutes, and when I'm done, I'm flexible like Gumby.
Something about being seated at that angle, with a small amount of weight on my heels stretches a part of my low back I can't hit any other way. My .02.