I was reading on internet about this few months back for someone in my family and found some really good exercises. Below are the few that we tried as well:
The Leg Raise (Lying): Lie flat on your back on the floor (or bed) with your arms at your sides, toes up. Keeping your leg straight, tighten your leg muscles and slowly lift it several inches. Tighten your stomach muscles to push your lower back down. Hold and count to five, then lower the leg as slowly as possible. Repeat, then switch to the other leg. Start with one set of four for each leg. This exercise strengthens the quadriceps, which are the large muscles on the front of your thigh that attach to your knee joint.
The Hamstring Stretch (Lying): Lie on the floor (or bed) with both legs bent. Slowly lift one leg, still bent, and bring your knee back toward your chest. Link your hands behind your thigh (not your knee) and straighten your leg. Pull your straight leg back toward your head until you feel the stretch. Hold for 30 to 60 seconds, then slowly bend your knee and lower your leg back to the floor. This exercise stretches and strengthens your hamstrings, which are the muscles on the back of the thigh that attach to the knee.
The Half-Squat: Standing with your feet shoulder-distance apart, stretch your arms out in front of you (hold on to a chair for balance, if necessary), and slowly bend your knees until you’re in a half-sitting position. Keep your back straight and chest lifted — don’t lean forward. With your feet flat on the floor, hold the position for five seconds, then slowly stand back up. Do 10 repetitions, and slowly work up to three sets of 10. This exercise strengthens the muscles in the front and back of your thighs, along with the gluteus (buttocks).
Hope this helps!