Increasing hip flexibility may help improve squat and leg press performance.
In this podcast (Quicktime ipod compatible, 19MB; Google streaming flash video), Bud Gibson and I (Nancy) demonstrate an effective hip flexor stretch that you can do before heavy leg exercises. This has been quite an effective stretch for loosening my hips and reducing stress on my back before we squat or leg press.
The first photo shows the proper start position for stretching the left hip flexor. I am kneeling on my left knee and my right leg is forward. Note that the angle of my right knee is greater than 90 degrees. Before I begin the stretch I make sure that my spine is very tall and straight. If I attempt the stretch with poor posture (slouching forward) it wil be ineffective.
The second photo shows the completion of the stretch. This, however, involves three distinct and separate steps. Each step should be fully completed before moving on to the next one. The first step (after making sure your back is straight) is to lean forward, leading with your hips, until the stretch can be felt in the left hip flexor. Once you feel the stretch hold it for a count of 2 or 3. The second step is to contract the left gluteus maximus. This will increase the stretch on the hip flexor and make the third step more effective. The third step is to tilt your pelvis anteriorly (imagine bringing the bottom of your pelvis forward) to further increase the stretch. The stretch should be held for aproximately 30 seconds and then repeated on the right side.
The third photo shows Bud preparing for the stretch. He is also stretching the left hip flexor as this is the knee he is on. The fourth photo shows Bud's surprise at the immediate effectiveness of the stretch. If executed properly you will definitely feel this stretch.