The treadmill is not a catwalk. "It seems pretty obvious, but I have seen gals rocking 6-inch heels at the gym in booty shorts and a tank top," says Bentley.

While we totally understand not wanting to show up to the gym barefaced, your gussied-up look may piss off serious patrons. "People show up in a full face of makeup, then spend an hour taking selfies without doing any work," says personal trainer Russ Howe.