When you take on more weight than you can handle, you're more likely to elicit eye rolls than applause from trainers. "If people are staring, it's not because they think you're cool," Thurman says.

"People will moan, grunt, and even fart a few times before they get a full rep," says Donovan Green, celebrity fitness trainer and author of "No Excuses Fitness." Which turns out, is all for nothing. "They'll either get no benefit or they'll hurt themselves," says Michael Spitzer, author of "Fitness at 40, 50, 60 and Beyond."

A not-so-secret side effect of going to the gym regularly is looking good -- but please keep your rippling physique to yourself. "Whenever I'm in the gym doing my beast-mode workout, I always see a guy who cannot stop staring at himself in the mirror," Green says. "It's like he is getting ready to ask himself out on a date."

McLean says that he especially can't stand guys who pull up their T- shirts to look at their abs like nobody is around. "Dude, the gym is full of people," he says.

Disrobing before a shower is inevitable, but a locker room is not a makeshift nudist colony. "Changing in the locker room is one thing, walking around in your birthday suit and sitting on the benches is another," Thurman says. "There are towels for a reason."

Want to get everyone to hate you? Rack a machine with a ton of weights, then leave them there for the next person to deal with. "If I could, I would literally pinch them really hard," says Green. "That is so disrespectful, and it shows no courtesy for fellow gym-goers, especially the ladies."

Smelling funky is one downside to exercise, but that doesn't give you free reign to show up stinky. "Sometimes people at the gym seem to have not showered or bathed for two or three days before coming to the gym," says Spitzer.

And yes, you might be able to get away with rarely laundering your jeans, don't be stingy about washing your sweaty exercise duds. "Some people wear the same clothes every time they are in the gym," says Kim Evans, a gym owner, certified personal trainer and fitness specialist. "Hey, buddy, you stink! Take your stuff home and wash it, please."