With your client's knee resting on your knee, have them press their knee forward into your knee at 20% of their strength for ten seconds. Then have them raise their knee (hip extension) and you can gently assist in this stretch at the end.
Psoas PNF Stretch from Deep Tissue Massage DVD
Psoas PNF Stretch from Sports Massage DVD
Psoas PNF Stretch Sidelying
Psoas PNF Stretch Variation Sidelying
Psoas Self PNF Stretch
See also the Psoas Page for other techniques to work with the Psoas Muscle