Previously we spoke about how the hip flexors respond best to soft tissue work, and thats true. However combine the soft tissue work with active stretching you will be on a sure path to success. This video below shows you how to actively isolate the psoas, and stretch it. You will notice in the first part of the video he is doing a bum exercise. This is important because activating the bum means through reciprocal inhibition that you are already starting to relax the psoas muscle and you will get much more out of the following stretch.