Iliopsoas can be a real problem. I’ve had severe problems with mine for 15 years. Initial massive trigger points shortened them up to the point that I had spondylolisthesis at L5-S1 and severe back problems that have just snowballed every since. It can bend you right over and cripple up your life if it gets out of hand and can become a permanent self perpetuating condition that is not only painful and disruptive but can even get so bad that walking any distance is a non starter.
I offer the following suggestions with the usual disclaimers- advice only, not a medical professional, do at your own risk, what does a guy on the internet know anyways?, etc., etc.
Look up Dr. Stuart McGill’s psoas stretch. It’s quite effective Also there are a number of lying off a table, bench or bed stretches that can hit it pretty good. Google image search shows quite a few. Experiment but be careful not to overstretch as it can take weeks to recover (at least in my experience). Frequent light stretching throughout the day is better than one intense session. Particularly after sitting for long periods.
What you are referring to as activating the psoas is direct trigger point work. Look up psoas trigger points and read up on it. Google Books search “psoas trigger points”- Travell and Simons Myofascial Pain and Dysfunction is the bible on trigger points. Volume 2 lower body should be one of the top results. Also The Trigger point Therapy book by Clair Davies has condensed practical application based on Travell and Simons. They will show you what you are dealing with and where to work, how hold the hands, etc…
Self work is done with doubled up hands back to back to reinforce the fingers but moving up and down the muscle belly. It is done while lying flat with legs down and then with problem side knee bent.
For iliacus work dropping bent knee out to the side while working closer inside the hip bone.
When you get a very painful spot that’s a trigger point. A hold for 30 seconds or so or until it “releases”- pain will subside and muscle will relax slightly. Then carefully stretch the muscle.
For active release hold the trigger point while flexing the knee towards the abdomen then slowly straightening the leg out.
Both sides should be worked. Even the side that doesn’t seem to hurt can have latent trigger points that will hurt when worked on and will cause dysfunction if left untreated.
For most people regular self work is enough to keep this in check. Be careful. Psoas pain and trigger point work can be extremely painful.
Professional massage therapists are trained to do this. Ask first as some are better trained and prepared than others at psoas work.