Horses have four natural gaits: the walk, the trot, the canter and the gallop. However, some horses are able to perform another gait called the "pace." While the trot is a diagonal gait, the pace is a lateral gait, with the front and back legs of each side moving at the same time. Most pacers are used for harness racing. You can't train your horse to be a pacer if it wasn't bred for it. If you do own a pacer, training it is very simple because the gait comes naturally.
Get your horse into shape. Before you can work on pacing speed, your horse has to be limber, flexible and in shape. Lunge your horse every day to condition it. When driving it with a harness, work on bending exercises to make your horse more agile and flexible.
Let the horse pace naturally. This should take no special command from you other than a cue to encourage the horse to go faster. You should train your horse in the harness rather than riding it, as the pace is an uncomfortable gait for a rider.
Use a hobble on your horse. Many harness racing trainers use hobbles on their horses. This apparatus, which is sometimes known as "Indiana underwear" (referring to pacing horses' state of origin), is a humane way to keep the horse in gait. Hobbles attach with straps above the tail and behind the withers (shoulders) and dangle around the legs to keep the horse in gait. Most racehorses use hobbles; however, you shouldn't introduce a horse to hobbles until you have the basics down.
Increase speed and muscle tone. This can only be done through months of training and practice. Be sure to allow your horse enough rest between training sessions. Consult a veterinarian or professional trainer about the optimum diet for a pacer. If you plan to race your horse, you need to give it some kind of exercise every day.