Lie down on your side. Raise the bottom leg up to 90 degrees (hip flexion). Place your top forearm onto the surface you are lying on. Make sure you hips are stacked on top of each other and your back is straight. Place the leg we are going to exercise directly in line with the back and hips.
To strengthen the gluteus medius, raise the top leg up as high and as far back as you can without strain. This should feel really hard for most people, at least to begin with. Try to keep the leg in this position for 15 seconds before bringing it back down again. Rest for 10 seconds and try again. Do this 5 times if you can. Repeat on both sides. Only do this once a day.
Important note: keep your hips stacked on top of each other and your leg slightly back (hip extension). If you move out of these positions you will start to engage other muscles more heavily, weakening the exercise. Again, if you are struggling, simply do less. Keep within your capacity, but do push yourself a little.