Here are a few starting points:
1. Learn about the psoas from the point of view of Liz Koch at coreawareness.com. She has dedicated more than 30 years to this muscle. You may enjoy her Yoga Journal article here or if you learn better via video/audio check out her video segments. What I've gleaned from Liz so far is that our psoas muscles are primal muscles. So they are the "messengers" of the nervous system, holding trauma and emotion. As such, releasing the psoas should be a tender and gentle process, as she teaches through her articles, books, and videos.
2. Consult a Women's Health Physical Therapist. They can evaluate for this and other areas of muscle tension that may be contributing to painful sexual intercourse one on one. Manual "trigger point release" of muscular tension can prove immediately relieving. If you find you identify with Liz Koch's view of the psoas, this myofascial release technique on the psoas in particular may be controversial. You'll need to balance the approaches for yourself.
3. Learn safe and gentle stretches for the psoas muscle. Find stretches from sources you trust online or take a yoga class. Your Physical Therapist can also teach you stretches. We teach you how to stretch the psoas and other hip musculature in our DVD guide: Healing the Pain 'Down There'.
4. Take care of yourself. You can't expect your psoas to be happy and healthy if you treat yourself poorly. Maintain good posture and sit on your sits bones. Breathe from your belly and not from your chest, allowing your pelvic floor and core muscles to expand and relax as you breathe in. Eat natural, whole, nutrient rich foods. Drink plenty of water. Move and play. Keep your stress under control: practice guided meditation, see a therapist or spiritual advisor if you need support coping with painful or traumatic events or circumstances (past or present), be nice to yourself and treat yourself to cuddles with a furry friend, a hot bath, a massage, a cup of tea.