I too have used a WebDecal to advertise my business. As was stated, you can sometimes obtain a good bit of response by just going to a mall or Wal-Mart or some other place that gets lots of traffic, and leaving your car sit there for a bit. If you have an extra vehicle and live close to one of these places, it might even pay to have someone drive you there, and leave your car with advertising sit there for a few hours at a time.
As for me, I drive about 8 hours a day for a company, so I get lots of advertising while doing my job. You just have to be careful using your company's vecihle to advertise your business, it could get you in a bit of hot water.