In this article, I'll help you start teaching your horse to halt square today, by relying not on less-than-reliable feel, but on 20/20 vision. (While you may not sense a mild, subtle, wiggly crookedness, you can almost always see one.) You'll use a two-word command--"stand up"--and three simple, low-key exercises that progress logically from grooming in the crossties (a place you don't ordinarily associate with square halts) to work in hand, and finally to riding. You can do these exercises at home and on your own--although you will need an observer on the ground when you start riding.
Exercise 1: Standing in the Crossties
You'll start to teach your horse that the verbal command "stand up" means "be quiet, be attentive, stand still and stand square, with weight evenly distributed on all four feet."
As you groom your horse in the crossties (or single tie if that's what you have), ask him to place his weight on all four feet (not necessarily square as yet), without resting a hind leg. Say "stand up" in a little bit of a deep growly voice as you demonstrate what you mean, so he makes the connection. If he's resting his right hind and you're on the left side, lean against him and push his haunches to the right until he puts weight on his right leg. If you're on the right side, use his tail to pull his haunches toward you.
Each and every time he stands on all fours (and there will be many, many times as he's learning), say "good boy." When he shifts or rests again after 10 seconds, calmly, quietly, and matter-of-factly say "stand up" and correct him again. With luck, he'll stand there a few seconds longer, and you can get a little grooming done. That may seem like a minor accomplishment, but it is an accomplishment; 10 seconds today is better than five seconds yesterday, and tomorrow it may be 20.
As he starts to understand that your verbal command means "shift a leg and bear weight on it," you can indicate which foot by tapping the hoof with your toe, or tickling or tapping the pastern joint with a crop. Don't whack or smack. When you start hitting, you make him tense and nervous, and standing still and square is about relaxation, not about anxiety.
Once your horse understands that his job in the crossties is to stand with his weight on all fours, explain that his job also includes standing square. By all means, lean and pull and tap to get him there, but in the long run you may have to bend over and pick up each foot and put it down where you want it (remembering to say "stand up" as you do).
From the very beginning though, teach him that moving his feet in a backward direction to square up, even by one step, is an unacceptable correction, because moving back is heavily penalized in competition. If he overcorrects and steps too far forward, or gets hopelessly cockeyed, back him a step or two, but only to rearrange him so you can bring him forward again (and only while he's in the crossties).
Some days, you will feel as if you're spending 45 minutes doing little more than picking feet up and putting them down again. When I reach the point where the horse is off just a little bit, I look the other way; I fix him only if he gets really sprawled out. Then I tack up and get on with my ride. Even though he's not 100 percent, I know he's starting to understand that something in his feet is not right when I say "stand up"--and I know he's ready to start in-hand work.
Halting Square in Hand
(Above) 1a. With the reins bridged evenly in my right hand and the whip angled against my left leg...
1. ...I establish a forward rhythmic walk. If my horse is sluggish, I reach back and tap his gaskin or hock.
2. To stop, I tighten my fist on the reins while bringing my upper body back and saying "Whoa."
3. If he stops with his right hind out, I say "stand up" and either reach my left hand back and under his belly to tap the leg or...
4. ...turn, swap rein and whip hands, and use my right hand.