If you have never used a professional automotive photographer, here are some factors you should be aware of and consider:
The Photographer: Make sure your photographer knows how to shoot cars, motorcycles, trucks, etc. Beware of photographers with lots of expensive equipment, but no real experience in photographing vehicles. Check out their advertising and website and see if they say anything about vehicle photography; or are they really a wedding, little league, or high school graduation photographer. Ask them what venues and clubs they've worked in, and what types of vehicles they have photographed. Most importantly, don't be afraid to ask questions and be comfortable with your photographer.
The vehicle: Have your vehicle in the condition you want it photographed. If you prefer it to be freshly waxed, chrome nice and shiny, have it that way prior to the photoshoot. If you want to show off your old Jeep and that you still 4wheel with it, get it good and muddy the day or two before the shoot. The more prepared you are, the more your pictures will impress
Background: If you already have a spot you want to photograph your vehicle let your photographer know. If you are unsure, ask questions. Good backgrounds are old garages, old service stations, old shops, barns, windy roads that go off into the distance, the options are limited only by imagination.