One of the most painful injuries I have had is a chronic hip flexor/psoas muscle problem. The hip flexor and psoas are the muscles that drives your legs up when you run, and, when they are hurting, running is next to impossible. The best hip flexor stretch and psoas muscle stretch involves kneeling on the floor with your affected leg in back of you. Try to keep your back straight as you shift your weight to your front foot. You should feel a pull along your upper quad/lower abdomen. Here is a picture of this hip flexor stretch:
When performing the hip flexor stretch, make sure that you only feel a light pull your psoas and hip flexor. Don’t overstretch! Additionally, make sure that you hold the stretch long enough to really loosen the problem areas. I recommended holding the stretch for 30 seconds, then rest for 30 seconds, then hold for an additional 30 seconds.
Back to stretching.