How to dress my cob to show?

I probably would plait him up if he has quite a long mane. As mane's can be distracting and get in the way of your hands in jumping. I'm not sure what class you mean by a pony club class - but if it's hack I would more DEFINITELY plait up. Personally I would do a running plait on his mane, the aim is to keep it tight and a consistant length - it both looks great and holds your pony's main neatly and out of the way. Is your horse more black or brown? That would probably make a decision on clothing as well, I think a black jacket with white jods always looks a lot neater and refined, which would be a good thing if your pony is quite built.