Some things to remember before showing:
-Have a halter that fits your horse. For an open show it doesn't have to have silver but needs to be leather or dark colored nylon. It does need to fit though. It shouldn't hang half way down his nose.
-Have a lead with a chain and run the chain underneath the horses chin and clip it in the rin near the crown piece. Having this under his chin will not hurt him, but it will polish up the appearance. If you run the chain over the nose it will change the fit of the halter.
-Clip him. Ears, eyes, bridle path, nose, underneath his chin and jaws. Also clip the white markings on his feet and around his coronet bands.
-Make sure that he his fit and clean. Riding him a lot should keep him fit and that will help the picture that the judge sees. Put baby oil on his nose, eyes, and his ears this will draw attention them under show lights. I will also take a little bit and put it on their bridle paths, it keeps it from looking flaky. Also, put baby powder or French White (chalk) on his white feet and any white markings on his face. Make sure that his tail is clean and free of shavings and band his mane. Even if you want to keep his mane long, it needs to be banded so that the judge can see a very clear view of his top line.
Anyway, sorry that I wrote a book, but this is definitely something I know about. :)
Good luck and more importantly, have fun!