Hygiene and manners are everything, and this definitely applies to the gym-going experience. Just because you're huffing and puffing on the treadmill, and losing the excess flab has you seeing in tunnel vision, doesn't mean that you can preclude the following golden rules:
1) If someone is waiting to use your machine and you still have a lot of sets to do, offer to "work in" with that person (i.e., you do one set, she does one set, and so on), as standing around and waiting for some bozo to finish up can be quite frustrating.
2) Use a towel to wipe down your machine. Sweaty traces are just icky.
3) After adjusting equipment for weight and/or settings, be sure to reset to the original position for the next person who comes along.
4) Leave the cell phone in the locker room. No one wants to hear your plans for the evening while they're in the middle of a grueling workout.
5) Don't stare at others. Yes, sometimes it's hard to resist the temptation to ogle hard bodies, but really--mind your manners.
6) Be aware of personal space. It's not unusual for a gym to get crowded, so when that happens, be sure not to bang into your neighbor with a barbell.
7) Don't wear anything with fragrance in it. Perfumey stuff mixed in with sweat and gym funk is an obvious no-no!