This discussion group is for questions about Yin Yoga and other body parts, such as shoulders, feet, wrists, etc.... Also, this is the place to discuss various conditions such as arthritis, osteoporosis, etc.
There are a couple of really juicy poses that can touch your hip flexors, of which your psoas is one: the Saddle pose is my favourite for this, especially when I bring my arms over my head and just lie on my back for ~8 minutes or so. But if the Saddle is not accessible, Swan and Dragons can also get in there.
Another mellower version is to do the reclining windshield wipers and just hold for 5 minutes on each side; to do the windshield wipers, lie down, bend your knees keeping your feet on the floor so you can touch your heels with your fingers and separate your feet as wide apart as you can, then just drop both knees to one side. Linger there. This works mostly the quads but there can be some psoas release too.