Here's a quick workout to help strengthen the ACL, PCL, and genu collateral ligaments. These are all really important ligaments in the knee.
While standing on one leg, jump forward 10 jumps, then backwards ten, to the right ten, the left ten. You'll want to try to really get some distance every few jumps. Do the other leg, maybe repeat a few times. This will teach your muscles to compensate for any undue stress from whatever activity you're pursuing. You're training your muscles to protect the ligaments. Take your time, warm up fully, and you're doing a fairly dangerous exercise (as far as gym work goes).
Do it at your own peril, aka don't blame me if you fall.
But feel free to thank me when it saves your ACL from a nasty fall.