There are a lot of ways to design car graphics but if text is involved then it becomes more critical. Some of the best examples of graphics merged with text are logos. A great logo will give hints about the company even if all of the text is removed. This should be your goal when creating any kind of decal for your vehicle or company vehicles.
When designing car graphics it helps to think metaphorically and geometrically. Thinking metaphorically will help you come up with ideas that, at first, won’t be that apparent. When the creator of the Nike “swoosh” logo went to work she probably could have gotten away with a graphic showing a runner in action. Instead she focused on the feeling of running. One rule of thumb to use when designing logos is that if the company is locally based then the logo should be literal or closely related to the occupation. If the company is more well known or national then it is all right to be more abstract.
Thinking geometrically can be very useful because often good design will involve geometry. Logo’s such as the one for AT&T do just that. Their logo looks like it was hatched in a physicists lab which is actually a good feeling to convey because they are, in part. selling physics.
A well placed graphic will add a touch of professionalism to your vehicles. Often do-it-yourselfers with put the graphic in the center of the text but it will often look better if it is put off-center. The key is that both the text and graphics be in balance and the the whole composition be harmonious.
Although most car graphics are born in computer programs, when you are just trying new ideas, it is ok and even easier to just use a scratch pad. Computers have not evolved to the point where it is easy to plop down abstract ideas and then scratch them out and turn the page.