It may not be a good short-term strategy, but for a long term improvement in sales, I would say you ought to try building a good reputation. Treat customers politely, be honest with them, and don't play stupid games.
For example, Bill Newton Nissan, of Conyers, Georgia, pulled the following stunt: When I bought a used car from them, they charged me a $250 fee to etch the VIN number on the windows. They claimed it was standard, even had it permanently printed on their boilerplate contracts. I NEVER found out where they'd etched the VIN - my guess is that they didn't and charged me anyway. While the total price was fair, I was ticked off enough that I'll never buy a car from them again - and as you can see, I also won't hesitate to tell others what sort of things they'd pulled. Was a $250 rip-off for etching worth losing any chance I would ever give them any repeat business?
I've encountered a few dealers with good customer service, too - ones who promised to throw in a part they didn't have in stock with a car who got the parts sent out the very next day, ones who were honest about a car's condition even when it was a total piece of junk, etc.
So the biggest piece of advice I'd give is that you shouldn't be a sleazeball.