Interschool where I am is so competitive, so I only do the handler classes and like bareback, sporting and sometimes I do jumping. Our interschool is in November, right when I have like 4 shows, so I don't know if I'll do it this year.
If you want to do like really well, well I wouldn't go all out with the ribbons. I'd do rosette braids instead of a running braid, and be careful what colour ribbon you put in his tail. You could put a red ribbon in his tail, just to keep all the little kids on ponies away from his butt.
If he has any white socks or markings, rub chalk or chalk powder into them. It will make them very white. Put lots of show sheen on him to make him nice and shiny and ride him like a pro.