What are your recommendations for building hitching posts?

There are numerous methods of tying horses. The most important factor in determining what type or types of tie methods you use is safety for the horse and the handler. If you are going to be training horses to be tied, a solid wall with two rings at a height of six feet spaced about three to four feet apart works very well. This allows you to tie the horse without being directly in front of the horse by running the rope through the first ring and actually tying to the second ring. The solid wall eight to nine feet in height prevents the horse from going over backwards, and may also help to prevent leg injuries.
Another good method of tying horses individually is with a six to eight inch diameter wooden post set in concrete. You can adjust the height at which you tie based on the horse’s size by tying at least as high as the horse’s withers or by bolting a ring to the post, making sure that the end of the bolt is recessed into the pole, not sticking out one side.
A tie rail can also be constructed. Set at least two posts (six to eight inch diameter posts) in the ground spaced six feet apart. Bolt two by six inch boards on each side of the posts. The tie rings should be placed on the posts and not the rails/boards. In most cases, horse facilities will have a combination of tying systems and have specific reasons for using each.