Knee Raises- The psoas major is a unique hip flexor that attaches to the femur and all of the lumbar verterbrae. With its attachments to the vertebrae, it has some control with the low back. If its too stiff, it can pull on it and bring it into hyperextension. If its too weak, the back will have less support and help from it, causing it to round easier. The psoas is most active in hip flexion above 90 degrees. Looking at this fact, guess where it might be of assistance. In the bottom of the squat. Where have I been weakest when squatting. In the bottom. A simple exercise like this can help a lot. I usually do them for 3-5 sets of 12-20 reps. I am using 20 lb ankle weights in the video but some people can start with just their leg weight and it will be plenty difficult. Notice how I focus on the top half of the hip flexion to emphasize the psoas.
P.S. I've been adding more videos to my youtube page so be sure to check them out!