- Smell normal - nobody wants to workout beside someone that smells like they have not showered in a few days.
- Wear clean clothes and shoes - clean clothes goes hand in hand with odor. Don't come in the gym wearing shoes that are tracking dirt all over the place.
- Re-Rack Weights - Don't be lazy put up what you used.
- Clean Equipment - After your done especially on cardio wipe down your machine. On weight benches and machines etc put a towel down.
- Talking - If your going to talk make it quick and during rest period in between sets. Otherwise your holding up the piece of equipment. I personally do not like to talk at all during my workouts. I'm their for a purpose and I don't want to lose my focus.
- Leave your ego at the door! if you can't lift it correctly don't lift it at all!
Feel free to add any others that I did not list in the comments.