Here is an effective method of releasing the Quadriceps and Iliopsoas muscle group.One of the best methods is with the utilization of a Yoga or Stretch Strap.  A belt or towel can be used as well.  The purpose of the strap is to give you the ability to relax the entire musculature of the legs and pelvis for the most effective release possible while targeting the anterior (front) structures of the leg and pelvis for a complete self stretch.
 

The basic application is to arrange the strap so that there is a small loop at the end of the strap, just large enough to fit over the mid-section of the foot, not the ankle. Lie prone (on your belly) on a bed or floor and position the foot with the loose end over their shoulder.
 

Before you perform this, practice how much, or little, force is needed to create an effective self-release. Feel as you bend your knee to the first barrier (or point of resistance). Feel how much is too much pressure, soften back to the first barrier and hold. I use the analogy of fishing. When you have a fish on your line, you must hold tight enough to engage the fish, but not so strong a pull as to pull the hook from the fish's mouth. Feel a release or two. Know that you may or may not feel the "release"; you may feel some softening, a prickly feeling under the skin, or nothing at all. But, at all times, hold the gentle barrier, never pulling the hook from the fish's mouth. Now take hold of the strap over your shoulder. Assure that your neck is comfortable, stay as relaxed as possible. Keeping the barrier engaged through the strap, holding for 5-8 minutes, or as needed.  It’s hard not to pull harder and harder, but tray to stay at the point where you begin to feel the stretch. If you experience back pain during or after the stretch, place a small pillow under the belly to rotate the pelvis a bit. Rest and repeat on the other side.

Experiment a bit with this technique. Performing this release with the thigh flat on the table, floor, or bed tends to concentrate stretch to the lower quadriceps. If you place a folded towel or pillow under the knee, extending the hip a bit, the emphasis will be more toward the hips and pelvis. I find this technique invaluable after a session of quadriceps releases, allowing the pelvis
to resume a more neutral position. You may feel the leg, hip and body move/unwind as releases occur. This is a fabulous technique for those clients with chronic knee pain, as well as anterior (front) rotation of the pelvis.  

Let me know how this works! Good Luck and Feel Great!