Rule #1
Keep Your Shirt On
If you're exercising indoors, you should be wearing a top. Some yoga classes are shirt-optional, but a lot of fitness clubs have (right-minded) policies against working out bare-chested. "Err on the side of caution and ask," says Herman Baptiste, co-owner of Asylum Gym LA in California. "If you can go shirtless, keep a towel on you to wipe up the sweat quickly."

Rule #2
Skip the Short-Shorts
Never wear high-cut running shorts—the only exception is if you're sticking exclusively to the treadmill, says Larry Twohig, cofounder of the Fitness Cell on Manhattan's Upper East Side. "Some guys have shorts that will ride up and expose parts that shouldn't be seen. You want them to hit the knee or slightly below."

Rule #3
Beware the Urge to Re-Wear
Change your shirt after every workout and your shorts after every two. Body heat during a workout can reignite a baked-in odor from your previous gym visit, so a sniff test of the gear you've stored in your locker doesn't always suffice. Gary Mahabir, a trainer at Clay in New York City, suggests choosing high-tech, lightweight antibacterial fitnesswear, which draws moisture away from your skin to reduce B.O.

Rule #4
Thirty Seconds: The time you should allow yourself to recover between sets on a weight machine. Taking any longer can make you a machine hog. If you need more of a breather, get up and allow others a turn.

Rule #5
Sip, Don't Snack
You may like to recharge with a PowerBar or an apple—just don't do it on the exercise floor. "I recently saw a member on a bicycle who pulled out a sandwich," says David Barton, founder of the David Barton Gym franchise. "That's gross on all sorts of levels." Stick to water or a sports drink.

Rule #6
Be Seen, Not Heard
"I have what I call the oversighers, who make borderline bedroom noises," says David Lynch, an instructor at YogaWorks in Los Angeles. While yoga involves a certain amount of group om-ing and deep breathing, avoid moaning and abnormally loud exhales. "I'm glad you're feeling good, but the whole world doesn't need to know," Lynch says. Likewise, grunting and throwing down your dumbbells relegates you to caveman status. "If you're dropping it, it's too heavy," says the Fitness Cell's Twohig. "Usually it's just somebody trying to show off."

Rule #7
Don't Misuse Mirrors
Club mirrors are for monitoring your form—save the vanity for home. "I can't tell you how many times I've seen people pop pimples in the mirrors," says Danielle Hopkins, a cycling and boot-camp fitness instructor at Equinox in New York. "Don't groom yourself on the gym floor. And checking out your abs and flexing in front of a mirror is just cheesy."

Rule #8
The Tipping Point
There's no rule requiring you to tip your trainer at the conclusion of a bundle of sessions or when you've reached a specific fitness goal, but chances are he would be grateful for it. Brandon Kolar of Equinox has had clients who have thanked him with restaurant gift certificates and stays at their vacation homes. He recommends checking with your club first. "Some gyms have a 'don't ask, don't tell' policy, others don't allow it."

Rule #9
Leave Your Private Trainer at Home
"We're there to service members using our facility, so it's disrespectful to bring in your own outside trainer," says Clay gym's Mahabir. It's also a liability. "The trainer you're bringing in is not covered by our insurance policy," says Christopher Cody Leake, a fitness manager at Crunch in San Francisco. Ask whether your gym allows members to purchase a day pass for outside trainers.