Psoas muscle attaches to your lower spine and to your thigh bone. If it's tight (sitting too much?) it pulls your lower back. It's one of the common causes of lower BACK PAIN! If you can stretch this regularly then your lower back may feel a lot better!
Tennis Ball Self Massage Technique - My massage therapist recommended this for the repeat-offender trigger point in my shoulder. It's amazingly effective (and, if you already have a ball, totally free). (Psoas Trigger Points Lower Backs)
To roll out your Psoas, lie face down on the ground with a ball on some books. Place the ball on the books in your abs above your hip to one side of your belly button. Relax over the ball and lie face down on the ground.