As a supple muscle, it requires no strengthening exercises, no 'pumping up', no force and no exertion. My kind of workout!
What it needs to stay healthy involves so much of what we do in yoga and what I love about it, release. It's almost impossible not to release the psoas while sitting in half- or full-lotus. But you don't even have to go that far. Sometimes just imagining your legs growing out of the waist changes how you sit in a chair or stand. Relaxation or corpse pose also helps but only if the pelvis rests evenly, with space felt between your back and the ground. We hear so much about 'tucking the pelvis under' and flattening the back when really this shortens, locks and potentially dries out this juicy muscle.
The psoas is juicy in other ways. I don't typically think of muscles as a major part of the nervous system, but the psoas is -- it's a key messenger. So by physically releasing this juicy muscle, we let the juice -- emotions, energy -- flow as well.