12. Don't be too chatty. A little conversation is OK, but remember that we're all there to exercise rather than socialize. Also, don't initiate conversation with people wearing headphones. They don't want to talk to you.
13. If you're male, don't ogle the females. They don't like that and you don't want to get a reputation as Creepy Staring Guy. If you must, take a quick glance using the mirrors then get back to work. Drooling is bad too.
I like the gym. I love it. It's my home away from home. I think the real secret to getting and staying in good shape is simply to love the exercise you engage in, and if you're fellow gym patrons are courteous then it makes loving the place a lot easier.