Here are 3 exercises you can do to strengthen your hips and help prevent knee injuries:

Gluteus Medius Strengthener:

Using an exercise band, tie one end of it around a sturdy object located a few inches off of the floor.  Stand perpendicular to that object with the band wrapped around your outer ankle (the foot farthest from the sturdy object it which it is attached).  Lift your leg out slightly at a 45 degree angle.  Hold for a second and release slowly.  Repeat this for 1 to 3 sets of 10-15 reps on each leg.  You will have to turn around to perform the exercise for the opposite leg.  
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Hip-Extensor Strengthener:

Using an exercise band tied in the same setup as above, lift you leg straight back behind you mentally focusing on reaching for something behind you with your foot, trying your best to keep the hips still while pointing the toes out toward the location you are focusing on behind you.  Hold for a second and release the tension slowly while lowering the leg back down to the start position. You may notice your toes touching the ground before a full 'drop' of the leg is achieved; this is normal.  Repeat this for 1 to 3 sets of 10-15 reps on each leg.

Advanced H.E. Strengthener Movement (and Notes for It):

The more you lean forward and down, the higher the ability to raise the leg will be and more gluteal development will occur.  Work up to this movement though, if you are a beginner or performing these movements for rehabilitation purposes.

Side Bridge:

Lay on your side with your forearm on the floor, under your shoulder and your feet stacked on top of each other. Tighten your core and raise your hips up off the ground, keeping your body formed in a straight line from your ankles to your shoulder. Lower your hips toward the ground but keeping them off the ground. To repeat, simply raise the hips up again. Repeat this for 1 to 3 sets of 10-15 reps on each side.

Advanced Side Bridge Movement (and Notes for It):

This move can work the abdominals, primarily the obliques, by adding a slight twist of the hips at the top of the movement.  The hips pivot slightly to face the ground, while the body is in the raised position.  At this exact point in the movement is where you will feel the obliques.  Then "un-twist" the hips before lowering back to the ground (as in original movement).  

In conclusion, All of these exercises can be performed with different band strengths; find the one that is right for you; move up to a more intense band when ready.  A Foam Roller, as seen below, also is a great device to adjust the Back, Legs, and Hips, providing comfortable pressure and possible pain release.