I don't have any first hand advice. I'm an inflexible mom to a super flexible daughter! I can suggest that you look at this web site for ideas. http://www.drillsandskills.com/forum/ They have a message board specifically for conditioning and stretching, and some of the ideas listed there will probably work for you.
I am etremely flexible! Try stretching after a warm bath; you're more flexible then. Strong hip flexers are important! Try pilates. Also try a move where you sit straight up against a wall. Straddle as big as you can and gently lift your legs up ten times. It'll be difficult for awhile, but keep doing it! It will REALLY help!!!!!!!!!!!