Everything JDI said! Here are a few more pointers from a judge's point of view. (I judge NFQHA, FQHR, ARHA, & 4-H)
Over the years have I have noticed, especially with youngsters that a lot of them tend to get a little dancey when you walk around their off-side. When they are little so much time is spent getting them to lead correctly that doing things on both sides seems to go to the way side a bit.
Work on moving all 4 feet independently and having a good set-up. More often than not that is the one thing that seperates the top of the class from the bottom. Example - Horse #1 stood still and correctly set up so it was easier to really look at legs. Horse #2 - stood parked out, or feet off-set, makes it much harder to critique legs and confo. Horse # 2 may have actually been the better of the 2, but without being able to see that in a good set-up, he knocked himself down a placing. It's even harder to judge one who won't stand still at all. I will do the dance briefly, but will not waste my time or the time of others waiting on following classes by slowing the whole show down for horses who haven't had the time spent teaching them the requirements for the classes they are entered in.
Practice trotting in hand A LOT! I will ask a ring steward to help get them moving if need be, but a youngster that will trot politely right on cue when asked says "They put the time in on this one."
Get them used to "showmanship switches" I also see lots of babies who tend to move trying to follow their handler when they try to switch sides.
Lots and lots of practice, practice, practice. Do short frequent sessions keeping in mind the attention span of a yearling and always end on a positive note. Good luck and hope you have a great show season!