In addition to those great tips, a few more.
First, if someone asks to "work in" on a weight machine, it means they want to take turns with you -- you do a set, then they jump in and do their set while you're resting, then you do your set while they're resting etc. It's always good manners to say yes, or to offer if you see someone waiting for your machine.
Second, don't ever interrupt someone in the middle of a set, unless the gym is on fire. I have a huge pet peeve with people trying to talk to me while I'm in the middle of a set. It kills concentration, plus if someone can talk while they're lifting, they're probably not working hard enough! Wait until someone finishes a set before saying anything.
Third, always put your weights and equipment back when you're done. Guys especially are guilty of loading huge amounts of weight on a machine and then walking away when they're done, or leaving DBs on the floor or BBs unracked. Even though you'll see it a lot, it's still bad manners.
Fourth, don't hog the equipment. Don't be the one who grabs three sizes of DBs and takes them all to your bench and no one else can find what they need. Use one size, put them back, then grab the next size.
Fifth, don't wear cologne or anything that's heavily scented to the gym. You'll reek when you sweat and it really bothers a lot of people when they're working out.
Most of gym etiquette is just common sense and ordinary good manners, so you'll be fine.
