The best way to avoid this problem (and to save water, plus be able to wash your car in the comfort of your garage) is to use this product:
http://www.driwashnguard.com/
Dri Wash N Guard was demo'd here in vegas at a PCA event, and I thought i would buy some (we have strict water restrictions). Boy am i impressed. Not only is it fast, but it cleans like no traditional car wash has ever done for me. Plus, it polishes at the same time. I have a 3x Black car, and I am telling you.... no matter how dirty your car is, this stuff works and it is safe (NO scratches, NO swirl marks, NO water spots). I will always use it.
I have nothing to do with the company, I am just a very satisfied customer.