Photographs by Brian Finke

Everyone hates a health-club vulgarian. Straight from in-the-know trainers, the code of conduct for the fit gentleman.

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."

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."

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.

4. KEEP RECOVERY TIME SHORT

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.

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.

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."

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."

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."

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.

10. HANG IT UP

Using your cell on the exercise floor, in locker rooms, or during a class is extremely inconsiderate (as is stepping over people and leaving the fitness studio for a chat). Excuse yourself only for important calls. "Take it to the lounge, front desk, or a conference area," says Kolar of Equinox, who has seen executives talking on their phones as they work up a sweat.

11. OFFER THE RIGHT ASSISTANCE

Polite: If you see someone struggling with a set, quietly and encouragingly ask if you can lend a hand. "It creates a sense of community, and it's easier than later having to give him CPR," says Clay gym's Mahabir.

Rude: Unless you're being paid as a personal trainer, keep tips on technique to yourself. For one thing, you don't know other members' goals and limitations. For another, you'll just look like a jerk. As Asylum Gym LA's Baptiste puts it, "Guys don't generally want advice from other guys."

12. LOSE THE BUDS

Wearing headphones to a class is insulting and distracting, says Equinox's Hopkins, who adds that you risk missing out on necessary guidance and fitness benefits. "Routines are often choreographed to the music an instructor has chosen."

13-15. PERSONAL SPACE, BY THE NUMBERS

Two arm lengths: The distance you should leave between yourself and the nearest exerciser while standing in front of the mirror.

Twenty-four inches: What you should leave between your yoga mat and the mats on each side of you.

One cardio machine: Minimum number of treadmills or ellipticals you should leave between yourself and the nearest exerciser if there are machines available.

16. CHECK BEFORE YOU CHANNEL-SURF

Don't just start flipping stations on a shared screen. Get the attention of the people in front of that TV first to see whether they're watching it, says Equinox's Hopkins. Even then, you're better off asking for help at the front desk. The club may have picked certain channels (music TV, morning shows, sports networks) to set a mood.

17. FEND FOR YOURSELF

In a packed class, saving a space or a bike for a friend is a serious faux pas. "That could be a source for a fistfight," Barton says. Graciously give the spot to the person who shows up before your tardy pal, who should understand.

18. PROTECT THE BENCH

"When you're changing, put a towel down on the bench," says Equinox's Hopkins. "No one wants to put their gym bag where you had your bare ass."

19. DON'T SHOW OFF THE PACKAGE

Avoid in-the-buff chitchat. And no matter how proud you are of your package, pack it away: Wrap up in a towel as you make your way to and from the shower.

20. BLOW DRY ONLY HEAD HAIR

Shaving naked at the sink is a no-no. Also, be a gentleman and refrain from blow-drying any body hair that isn't on your head.

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