Hip Flexors are not a big part of the body or a muscle if you will but relaxing the hip flexor is very important for increasing flexibility and strengthening your legs for stronger squats. You can do this right before heavy lifting. In this video, Brian Klepacki from Criticalbench.com will instruct a simple exercise to stretch the hip flexor with a foam roller.
First, you need to put your right hip flexor which is just above the quads or your hip bone on the foam roll and start moving back and forth. By doing this the hip flexor is relaxed. In the second step, sit on the top of the foam roll, shoulders go right back on the ground. The only body part above the ground is the hip via the foam roller. Then pull the left knee into the chest, all the toes should point straight towards the ceiling. You will feel a tremendous stretch in the right hip flexor. Do this for 20 seconds.
Do the same thing with the other hip flexor. Put the left flexor on the foam roll, move back and forth then lie on the foam roll and pull the right knee into the chest.