Get a light stretch band (available at www.jumpstretch.com), sit in a chair, and place your legs inside the band. These things are basically just big rubber bands. You might need to fold the band once or twice to tighten up the slack. Move the band up your legs, so that it is right below the knee. With your feet staying planted at about shoulder-width apart, rotate your knees outward (like attempting to do a split) and then bring your legs back together. repeat...etc.

This is what many powerlifting gyms use to strengthen peoples knees, so they don't bend in when squatting heavy weights. If it helped these guys squat 1,000 lbs, then it should help you as well.