Do you carry around a duster? Can really help to smarten up a horse quickly with a quick brush-over when you get a spare moment.
Also, don't be too uptight when performing but allow yourself to smile but be careful that you remain respectful - a showing is supposed to be mainly on the horse but a judge is also human and will most probably be more predisposed to like the horse if she/he likes the rider! Be careful of the judge though; they expect respect and so some many not appreciate a bit of life and interpret it wrongly.
Find your own spot in the ring, for flat classes. Remember, equitations judged on YOU not your horse, so work on your position. And if you're really concerned, you could ask the judge after your class what s/he liked and see if they have any suggestions on what you could work on.
For your flat classes you really have to get the judges attention since there aren't just one person in the class at a time . If some horses are getting too close I'll wait till we get around to the judges box and make a circle right infront of the judge ...seems to work for me