What are the best exercises for improving flexibility of the hip flexors?
How often should they be done and for how long?
Ok I had a hip issue a couple of weeks ago and this one did it: Get in a lunge position…assuming you know what lunges are. But, extend both legs as far back and forward as possible till you feel the stretch. When you feel it…hold for 20-30 seconds. I just rode 42 miles and ran 5…I think I am going to do it now. Thanks for motivating me so I don’t get too tight there again.
http://www.yogacards.com/yoga-postures-2/kapotasana-pigeon-pose.html
feels oh so good… hope this works for you
Try This:

push forward from the hips. you’ll feel the stretch in the back leg.
i try to keep the front leg at 90° or less. you can also do it with the back leg on its toes, foot in, knee not touching the ground.
also, you want to make sure that the surrounding muscles are loose, flexible, etc.
-charles