In chronological order:
1. The biggie: Worked on upper-body posture. I used to swing my arms way out in front of me and and side to side, causing a twisting motion in my torso that resonated down through my hips, knees, right to my ankles/feet. That caused all kinds of problems and pains, over a number of decades. A running coach worked with me a while and got that sorted out and opened up a whole new world of running for me. To fix it, all he had me do was bang my elbows into his hands as he stood behind me (well, that's the simple version).
2. Started including barefoot sessions in my sessions. At first just a few hundred meters at the end of a workout (this was around 2005, inspired by discussions at the old Cool Runnings website). At the same time, got the F out of the "stability" shoe loop that I'd been caught up in since about 1978 and into "neutral" trail shoes (Inov-8s) because I:
3. Got off the roads and onto the trails. This was an important change because it added another three or four dimensions to my running, made it more of an holistic/whole-body experience.
4. Ditched the shoes altogether. This also encouraged me to finally get serious about the higher turnover/shorter stride techniques (which have been around for decades, btw).
The list could go on for a while and would definitely include strength training for quads and hips.