Gotta have a cool logo and get an affordable vendor to make T-shirts (if you're going THAT far). If you can only afford 1 or 2 colors for a silk screened design, it'll severely limit what the artist will have to work with. I would go with a vendor who can do multi-color designs and offers white or light-colored shirts with sizes up to 4XLT (a lot of car guys are old & fat tall guys, after all rofl ).
The logo will help with the flyers, too. Make sure you have something simple enough for small print (on flyers, as web page graphics, etc.), or have a 'portion' of the overall logo for that, or an alternative small logo.
I mention this because I get tagged for designing the logo stuff for the Goodfellow AFB annual Car Show stuff every year - and the people at the Arts & Crafts Center always make my life a living Hell because of only allowing 2 colors for the silk screening on the worst possible color of shirts, and it can only be so big, but has to accomodate pretty much everything about the car show (hot rods, muscle cars, trucks, motorcycles, boats, etc.) and then wind up altering the design somehow - and they never seem to manage to make a 3XLT for me... despite the fact that I designed the dang thing (I typically get stuck paying the registration fee for an XL shirt that winds up polishing the hood of my Jeep).
Another thing our car show coordinators do is to hit the local auto parts stores, online vendors, and speed shops for 'freebies' & 'give-aways.' Everybody who registers a car gets a grab bag full of things like automotive stickers, calendars, a hat, car care products (trial-size, usually), coupons for a free drink and a hot dog or 2 from a local concessioner, a pre-printed 'placquard' to fill-in with car information, a raffle ticket for door prizes, a ballot for all of the categories, and a car show logo'd T-shirt.
Our voting is from among the registrants, with the Wing Commander picking the over-all winner. But we've been complaining about there needing to be a 'People's Choice' award, where spectators can vote as well.
The success of your car show will depend a lot on what the average non-trophy winner will get to take away - they'll be back next time if they feel like they got their money's worth.
Edit: added
Categories at our show (makes for a lot of plaques/trophies):
Best of Show - Wing Commander's award
Long Distance Award
Best of Class 1900-1929
Best of Class 1930-1939
Best of Class 1940-1949
Best of Class 1950-1959
Best of Class 1960-1969
Best of Class 1970-1979
Best of Class 1980-1989
Best of Class 1990-Present
Best Motorcycle
Best of Class Motorcycles - Touring
Best of Class Motorcycles - Sport
Best of Class Motorcycles - Custom
Best of Class Boats
Car Clubs often show up in force, so make sure to recognize those clubs, and let them present club awards as well
Hope that helps!