My friend owns a Clydesdale mare and she used to take him in halter classes as well. I had the opportunity to help her out one afternoon, before a show. For the mane and tail, I suggest using conditioner when you bathe him, to give it a shine and to bring out some of the snarls. We braided her gelding's mane and tail, but it's not required for some shows that the mane and tail be braided. It looked good to me, though!
Just remember to keep his feet lined up and standing still when your in the ring. Judges will usually take off points if the horse is moving all over the place, or his back feet are not aligned.
I also suggest getting him to lower his head, but dont drag it down when the judge is coming near him. You shouldn't have to force him to lower his head, and judges like to see a relaxed horse.
Any other questions?
Sorry if I'm not very good at this - I haven't been to a show in a while!