The first step is to review your exercise program. The idea is to strengthen the quadriceps muscle along the top of the thigh and get the kneecap tracking straight up and down. A balance between quadriceps and hamstrings muscles is essential, too.
If you aren't already working with a physical therapist, now would be a good time to seek out some advice and guidance. The therapist may give you a device to take home to stimulate the quadriceps muscle. It's a form of treatment called electrical muscle stimulation(EMS).
It takes about six weeks of consistent use to see a difference. You can expect decreased pain and increased pain free motion.