Use colors such as white and black (like a checkered flag), and have fun mixing in other colors.
Hang up posters of cars (if strickly a car theme party). You can get some from brochures from different car manufacturers. For a Nascar race car party, use posters of favorite race car drivers.
Before the party, find a large appliance box and cut it out in the shape of a car. Let the kids color it when they arrive.
Have stickers to put on small toy cars.
Design your own race car logo.
If playing any games on teams, divide into car colors - red, yellow, white, or call the teams by different pit crew names.
Furnish the kids a pinewood derby car (already assembled) and let them paint it or cover it in stickers (you can then race them as a party activity).
Have lots of toy cars out on the floor, with a map drawn on paper on the floor (or use one of those rugs that looks like a city) and let everyone play!
Make a cake in a figure 8 shape, and cover with chocolate cookie crumbs (oreos work great) to look like a road. Include a lane stripe down the center, and decorate with some favorite small toy cars.
Make a rectangular cake and place some small toy cars on top.