For clothing I suggest:
Black tall boots, breaking them in is really easy, just use a ton of leather softener and stand on stairs. If you aren't use to them then I would ride in the a lot before showing in them, they have a different feel.
Navy or charcoal jacket and make sure it is fitted right. The back should not be too tight when you put your arms out and the bottom should hit you in the right place, a baggy jacket will make you look unpolished. When you get your number loop the string through the middle hole, tie a neat bow and tuck the bow between the buttons so that it is completely hidden (it looks a lot better than having the bow show). The shirt should be a subtle color, either solid or with a simple patten. I like monogramming my collars (with a color that matching the jacket), but that isn't needed. Pants should be beige or a greenish/beige-ish color and they can be slightly baggy. Your Helmet should be black.
I don't really have that many tips except try and stay away from the pack so you don't blend in too much and never pass right in front of the judge if they are on the side (they can't see you when you do that), take the inside track to pass them. When you line up try and get a spot in the middle, so the judge has one last chance to remember you.