If there is enough light to allow it a CPL helps a great deal with unwanted reflections. Show cars are usually highly polished and reflections are everywhere.
A good diffuser on a speedlight also helps a great deal.
Look for details and unusual views. I've shot a lot of car shows and of all my shots my personal favorites are the detail shots. Reflections of other cars in chrome, emblems and logos, injectors and blowers, things like that are to me more interesting than the overall car sometimes.
Respect the effort that went into them. I don't think I've ever been to a car show that I didn't see someone leaning on one of the cars to get a better look. I never, ever, touch anything at all.
Don't forget to watch the people. Sometimes the most interesting thing at a car show is the people looking at them.
Don't forget to have fun!