Everytime he moves, just put him back and make him stay. If he moves again, just put him back. Someone once told me never to circle your horse out of line because that teaches him if he moves, he wins and get's to circle. Don't back him up either, that shows frustration to the judge. Just set him up again. And again. And again. Like PB said, just practice practice practice! Everywhere, square him up and make him stand. Literally, everywhere.
As for pivoting, i've posted about this before, so I'm going to copy and paste. Sorry if some of it isn't relevant, just disregard those parts.
I usually start the training by just having one hand on the halter, and hold onto the noseband of the halter, with the lead rope in that same hand. Then take your other hand and put it right in front of the shoulder, on the horse's neck. Your body language is what alerts the horse to what you want, so EVERY time you want him to pivot, point your belt buckle at his nose. Every time. Then he know the difference between that and you asking him to back. Then walk toward the horse and push on the neck and lift up just a tiny bit on the halter, while also pushing gently with that same hand. (Sorry if that was confusing... I don't know how to explain over the internet haha) As soon as the horse takes a step, release ALL pressure. I repeat, ALL pressure. That part is imperative. Let the horse think for a minute, then do it again. When you get to the part when your horse has to pick up a back hind leg to continue the pivot, which will take a while, make sure you give him a cue for that. For example, a little bump on the halter might mean "Okay, now you pick up that back leg." DON'T release pressure unless the horse takes the step you want. If you don't give him a cue for that, he might do it in the arena, he might not. If you give him a cue for it, then he'll do it whenever you give him that cue, so the timing lies in your hands, which is a better place for it than in your horse's mind haha. Eventually, you should be able to move your hand away from the shoulder and just use your lead rope. Good luck! Just ask if I was confusing or if you have any more questions! I love halter and showmanship... So much fun!