While it's true that a pretty horse catches the judge's eye, your grooming and trimming efforts are going to be wasted if your horse is not capable of performing the tasks required in your chosen competitions and classes. In fact, taking stock of your horse's good and bad points should be an important part of your decision which classes to enter.
This means you have to do a lot of thinking and planning before loading up the horse trailer and heading off to a show.
For example, if you're planning on entering a hunter class, your horse should be able to jump the specific height of jumps that he will encounter, he should be able to canter smoothly and calmly, changing leads when necessary and adjusting his stride without it being obvious to the onlooker. If your horse can jump the height, but doesn't have the form for hunter classes, you might wish to try jumper classes, or even horse trials or eventing if he performs well in the dressage arena.
If you're planning to enter a dressage class, he should be able to perform all the movements required at the level at which you plan to show. It's generally accepted that riders show one level behind the level they train at at home. While your horse is learning to balance during movements like a ten-meter half circle, or leg-yielding at home, enter him in Training Level classes that don't require those movements. Wondering whether he's going to blow up when you ask for a certain movement isn't going to help your show nerves.
In Western Pleasure, the horse should go at a nice steady pace, with a level head carriage on a loose rein. If your horse has a need for speed, you might consider barrel racing, or giving him something to think about by training for, and entering trail classes.
Several weeks before you intend to start showing, you need to start working on your horse's level of fitness. If, for example, you've only jumped individual jumps at home, taking him to a show and expecting him to jump an entire course in one go is going to stress him. If you know your horse gets agitated when in the arena with other horses, practice at home with your friends to get him used to the activity around him.
Knowing exactly what will be required of you, and knowing also that you and your horse can do it, will put your mind at ease about the prospect of your upcoming show. This means--practice, practice, practice!
And most importantly--HAVE FUN!
Preparing for Show Season Page 1, 2