I pulled out my copy of of Arnheim's Principles of Athletic Training to see what it had to say. I'm only going to give you the stuff related to strengthening/stretching, but it also lists a bunch of other therapy stuff you should be going through concurrently--if you want to give yourself the best chances of recovery, you should see a physiotherapist for rehabilitation.
First of all, it says that after a groin strain (which includes a psoas strain) exercise should be delayed until the groin is pain free, so make sure you don't get started up again too soon. Once you're ready for exercise, your program should focus on gradual restoration of the normal range of motion. It also says that until normal strength and flexibility are regained, you should wear a protective brace like one of these.
Here are a couple stretches for the hip flexors.The lunge and assisted stretch will target the iliopsoas. The stretch with the ankle held will target the quads (the rectus femoris works with the iliopsoas as a hip flexor). When doing the lunge, try to keep your pelvis neutral; if you tilt it anteriorly (forward), it takes away from the stretch. If your other hip muscles have become tight due to lack of use, you should probably stretch those too. Exrx has quite a comprehensive set of stretches for the hip muscles (right column).
The book has a sample injury management plan for a basketball player--not exactly the same as ultimate frisbee, but the running and direction changes are similar. Two to three weeks after the injury, they recommend doing resistance training once a day (10 reps, 3 sets). When doing normal weight training you're supposed to leave at least 48 hours between workouts, so once you become stronger and start increasing resistance, I would stop doing this daily. Always remember that it shouldn't hurt. A cable standing leg raise would work the iliopsoas (See ExRx for more options). Just like with stretching, you should target the other hip muscles as well--you probably haven't been using them as much due to the injury, and training the iliopsoas alone would create imbalances (same ExRx link as immediately above, just scroll). Other muscles that help the iliopsoas are the quads and abs, so you may want to focus on them as well.
To get back into running, the book suggests you start in the pool:
Jogging in chest-level water (10 to 20 min) 1 or 2 times daily for first exercise rehabilitation week [2-3 weeks after injury] followed by flutter kick swimming (pain free) once daily during subsequent weeks.
Water provides quite a bit of resistance, so it helps you build strength. Three to six weeks after the injury, you may be able to get back into jogging:
Begin a program of jogging on flat course, slowly progressing to a 3-mile run once daily and then progressing to figure eights, start with obstacles 10 feet apart and gradually shortening distance to 5 feet, from one-half speed to full speed. (link added for your reference)
The figure eights will prepare your muscles for the direction changes in your sport. Applying cold to the area after exercise can also help prevent inflammation (excess inflammation slows down recovery).
It sounds like you're outside the time window for this specific recovery time line, but most of this stuff is still probably worth trying, especially if you didn't do it within the first 6 weeks after your injury.