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."

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.