Hi Sheila! I've had this problem for years and it's a real pain!
I saw an orthopedic doc at least 10 years ago, and did some PT for it. What it all boiled down to is you need to keep your quads in good shape. They hold the patella in the right place. In addition to a regular legs workout, I do isometric exercises, which conveniently can be done in bed! Just tensing and holding quads for 10 sets of 10. In the original PT they also had me doing those leg lifts/lowering you may remember from HS. I've just begun doing more squats which should help too. Between my knees and being overweight, I find it hard to squat down to do anything like weed the garden, or rummage in a lower cupboard. And I suspect I'll never be able to do anything that requires me to actually put weight on the patella due to the damage - but hey, who needs to crawl around on hands and knees anyway?
As to braces, I've used the ones they make for volleyball players, with the pad removed, with some success. And I avoid anything like some aerobics moves that require you to land on your leg with the knee twisted - yowch! And my last piece of advice - keep moving. My knees get really stiff if I'm sitting more than a hour, like happens at work all the time. I need to remember to get up and move around at least once an hour.