The treatment includes a wash-and-dry, a clay paint decontamination treatment and a buffing on all paint and chrome.
To enter, BMW owners fill out a brief form. If they indicate that they are in the market for a new or used car, they receive an additional entry.
Winners are chosen randomly.
The dealership's Facebook fan base has grown 50 percent since the program's inception in May 2014, Chris Markey, BMW of Austin sales manager and Internet director, told Edmunds in an e-mail.
"It started because Central Texas was in a severe drought," Markey wrote. "The City of Austin implemented water-rationing plans that meant not wasting water on things like car washing. So we came up with a 'waterless' car-wash method and wanted to give back to our loyal customers and decided to give away a Spa Day monthly to one of our lucky Facebook fans. It has been a hit and is very popular."
While he can't say if specific car sales have resulted from the giveaway, Markey is certain it's helped.
"The winners tell everyone they know and that benefits the store in a variety of ways," he wrote.
Edmunds says: Keeping customers happy is all in the details, and the detailing.