Have a towell with you and wipe equipment after use. Do not talk to people in the middle of their set. Avoid walking directly in fron of someone during their set. Keep your personal hygene in check. This goes for body and clothes.
Nothing is worse than week-old smelly socks, shirts, and shorts.
Keep profanity in check. Yelling during a lift is ok. The one rule I follow and enjoy others to follow is the second person on the equipment is requires to place the weight where the first person left it. He was there first. This list is not all inclusive and I would be interested in other people's suggestions.