I'll take the obvious answer and suggest that you invest in the Magnificent Mobility dvd if you haven't already (but if you already have it, then you just need to review it and put it into action.) Cressey and Robertson did a seriously great job putting that together. They showed 30+ movements of varying difficulty and I've made four of them staples in my warm-ups every session. Night and day difference.
If it's a weakpoint, then yeah, I'd say it deserves some specific, dedicated attention. If you have a partner, you can get even more funky with some PNF/contract-relax stretching. But even if you're solo, there's a bunch you can do with a little planning. Just remember that if you're doing a stretching-only session, you still want a decent warm-up beforehand.
Fixing the hip flexors and getting them nice and mobile is going to make a world of difference. I like all sorts of lunge-position variations to target the area, but I'll also sometimes use a back kick/reverse leg swing-type of movement for a more dynamic drill.
I've always liked static stretching as a "cooldown" and to help ward off soreness post-weight training, but it's not the best for flexibility gains. That's going to be more a contract-relax/PNF type or something more dynamic.
Also, I just thought I'd add Alwyn Cosgrove's take on stretching, from his book "The Secrets of Martial Arts Conditioning":
So, um, yeah.