1. Turn the reference picture upside down. When you look at something in an abnormal way, your brain has to register it differently and as a result, you notice things about a picture that you wouldn't previously.
2. Once you think you're done the drawing, hold it up to a mirror. Mirror images show small mistakes that our brain wouldn't ordinarily perceive. Are the eyes slightly different in size? Looking at it straight on, you might not be able to tell. Looking at it in a mirror, you'll see the mistakes quite quickly.
3. Split your subject into geometrical shapes. When I'm drawing a horse, the head is made up of two circles (the forehead and muzzle) joined by a rectangle. The ears are two triangles. The body is two circles again (chest and hind quarters) joined by a rectangle. Each leg is two circles (knee/hock and fetlock) joined by ovals (forearm/stifle and cannon/shank). The hooves are squares. Once you have the basic shape, you can begin fleshing out the actual shape of the horse, adding curves, connecting the head to the shoulder, making the entire shape look less 'geometrical'. The drawing I did below is a good example... it's not quite finished, but you can kind of see how I used geometrical shapes to piece it together.