Good tips via the link provided in that first reply.
Make sure your camera is balanced horizontally and your horse is well balanced within the viewfinder to start. Aim for a spot just behind the elbow and set yourself and your camera directly across from it both vertically and horizontally. If you are too high, your horse will be too much body and if too low, too much leg.
Once you get a feel for it, and photographing your horse (in particular), you can start to make minor adjustments with your aim to help minimize weaknesses and maximize the strengths that your horse may have.
Practice, practice, practice!!