How about touting your quality, such as;
We get your whole car clean the first time, or
We clean the areas other leave behind.
We have a small chain of washes here which with its name you would think that they really get wheels clean. That however among other things, is their most glaring weakness.
Now as operators, we all know you can't turn a silk purse from a sow's ear, but it is really amazing to me how many operators turn out a really mediocre product. Between wheels and fronts and backs of vehicles, its easy to see why they absolutely die during the off season. Regardless of the money spent, people always seem to expect quality. A continuous failure to deliver that earns you a bad reputation. of course these same operators make arguments like, "What do you expect for $3," or "Its Ok, people just want to get the salt off." People are consumers, they remember when they get poor service or quality is lacking. It is just as important to put out a great product on a 1,000 car day as it is on a 100 car day. Its called consistency and its what brings people back, makes them loyal and gets them to refer you to other people. After all, when was the last time you referred someone to a business with the recommendation that the service was ok but the overall quality sucked. Answer, never. Superior service and great quality are the bullwarks of any successful business. Omit these and yes, you'll wash some cars - when people don't have a choice. But when the weather breaks they'll be washing their car at home.
As a consumer, I never understood the concept of low price and mediocre quality. People are people and they will remember the bitterness of poor quality long after the joy of low price is forgotten.