Longeing is not only useful in teaching the horse rhythm, balance, bending, and obedience, but is also useful in helping the rider improve his position and balance. First, tie the reins around a clump of mane and take off your stirrups before you begin. Make sure the horse is equipped with side reins and that the person longeing you has a longe whip and is familiar with how to use it, for the ground person will be controlling the horse while you concentrate on yourself.
Try working on the longe line at the walk and trot first, until you feel really comfortable. You can practice relaxing your seat so that it follows the horse’s back at the sitting trot, or can work at the posting trot, holding your hands in the same position as if you had reins, but finding your own balance without having them in your hands. You can concentrate on feeling the center of your horse, so that you won’t be leaning to one side or the other inadvertently. Then you can add a little canter work. The entire session should not last more than 20 minutes, and the horse should be reversed every five minutes to keep your work even on each side.
You can find numerous longeing exercises in my book, The Complete Guide to Hunter Seat Training, Showing, & Judging, pages 38-46 and 171-173. There is also basic information on longeing a horse in the sample chapter on this website, at http://annamullin.com/book/sample-chapter