Standing Hip Flexor Stretch
Start the stretch by placing one foot on a chair. The foot that stays behind should be in a slightly "pigeon-toed" position. Now, allow your front knee to bend slightly and reach both arms up overhead as high as you can go. You should feel a stretch in the front of the hip of the leg that you're standing on. While keeping your arms overhead, reach them slightly to the side - away from the hip you're stretching. This will increase the magnitude of the stretch so proceed with caution. You should pivot from the hip joint, not the lower back. You should NOT feel any discomfort in your back, knees, or anywhere else for that matter... only a pulling sensation in the front of your hip. You may feel a little stretch in the buttock of the leg that is up on the chair.
Note position of hands - reaching high overhead, and slightly off to the side. Hips will stay facing fairly straight ahead.