Basically, as forward as you can get them. I have T-rex arms but still above or in front of the withers is where I try to get them. They should stay within 2-4" above the horses withers, whether they are beside the neck on a dressage horse, or above the withers slightly on a h/j horse. They should not be raised too high and should remain quite close together so the reins are contacting the horse's neck on both sides - at all times.
Google Robert Dover, Tina Konyot and Edward Gal for pics of good hand placement, and position in general. I'm sure George Morris has a book written on the subject as well lol.