If you are one of those people who shows up reeking, don't try to hide it with perfume or cologne. "The smells will radiate as you work up a sweat and may cause nausea among your fellow gym-goers," says Thurman. "And if you think wearing your commercial body spray will have the ladies lining up, you are sorely mistaken," he adds.

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.