One thing I learned early on: there are many things that need to be right with a dressage photo. Rider position, facial expression, horse leg position not quite right, and hundreds of other factors. This is of what makes specializing in Dressage such a challenge!
In going for a head shot, many of those factors are removed. It's still tricky, the mouth needs to be closed, head pole correct, ears up or at least in the concentration mode. In doing action head shots, I try to get what I am looking for before the horse is in the arena and circling the outside. Usually the ears are more alert as they are eye balling their surroundings. Following the horse with the camera, many interesting angles can be achieved.
Seen here is a photo of Favory IV Delta, a Lippizan Stallion owned by Deborah and Marvin Hausman, Quailhurst Farm in Sherwood, Oregon.
Search the Phelps Photo Data Base for more head shots. Click on search by rider, then select from the position field drop down list, head shot.
For photography enthusiasts one of the oldest and best sites on the WWW is Philip Greenspun's Photo.net