With the client supine, have her bend her knee and put her foot flat on the table. Then, after warming the area, sink in with your fingertips about halfway between the navel and ASIS. Sink slowly and gently, but deeply. You will feel the muscle - have the client tense it, if you are unsure. If she lays her leg down flat (slowly!), you will feel the psoas stretch out, and it will contract as she raises her knee back up. I start with massaging along the muscle fibers, then move to going across the fibers. It can be stretched by doing a cross-hand stretch, with one hand on the lower ribs and the other on the upper leg.
Another good thing would be to stretch the QL's. With client supine, grab her ankle and gently lower your weight into pulling the leg. When you get it fully stretched, have her do a "hip hike" move - moving her hip upwards towards her ear. Have her hold that 10-15 seconds, then have her gradually relax while you continue to hold the stretch. Both legs, both supine and prone, will give the QL's (and a few other muscles!) a really good stretch, which can greatly relieve low back pain.