Stretching is very important, as is strengthening the hind end to take weight and pressure off the stifle. Regular equine massage will also prevent build up of tension and prevent pulls and strains. It can also reduce inflammation and speed healing in conjunction with previcox or on its own depending on severity. Good luck and hope this helps.
I've had luck with previcox, building up muscle in the hind end and then having the stifle blistered if neither of those help much.
I had a gelding who had ocd of the stifle and I had the stifle surgery done on him. He didn't seem 100% but putting him on previcox has helped a lot years later after the surgery. He is rideable as a lesson horse.
I have a 9yr old ottb that ended his racing career due to a stifle injury. Depending on the severity of the injury will determine what you need to due. With my gelding we did extensive stretching before each workout. We walked up and down hills and walked over hay bales (which allows him to lift and stretch the leg naturally). He also needs alot of turnout time (stalling makes it stiff and can even cause it to lock). If all else fails talk to your vet about surgery. My gelding will eventually need stifle surgery due to how easliy his locks and causes extreme discomfort and even prevents him from moving some times. The vet will go in and cut the stifle (or so Ive been told). The vet said he will have a completly normal life but may move a little off. This will eliminate all of his stifle issues.