The unwritten rules of the weight room can be quite daunting for the uninitiated. I have spent years learning the subtle often confusing code of the gym. I am by no means an authority on the subject, and still have much to learn. I welcome any other insight into gym etiquette as this is by no means a comprehensive list. Thank you.
Always put your weights away, this seems basic but others will appreciate it. A clean gym is a safe gym.
Try to limit how many pieces of equipment you are using. We all love circuit training but taking up half of the machines in the gym will make you no friends.
If anyone asks if they can "work in" with you, they are asking if they can use/share the piece of equipment with you. The answer is always yes! Most will respect the pace of your workout and "work in" when appropriate.
Never curl in the squat rack, this is a long standing well enforced rule. If you want to do curls there are many options, but if you want to do squats there are few. Leave the squat rack for the people doing legs.
Never block anyone's view of the mirror. This one is less about vanity and more about lifting form. Looking at yourself in the mirror at a gym is very helpful in keeping good form. That is why gyms have so many mirrors. This can be difficult when the gym is busy, but people will appreciate the effort.
Don't skip or jump rope in the lifting area. This takes up a lot of prime real estate, there are usually more appropriate areas for this like a basketball court or cardio room. Same goes for abs, find a quiet out of the way place.
Please don't use chalk unless you are an Olympic lifter going for a personal best. Chalk makes a mess and is usually not needed.
Hygiene, this shouldn't need an explanation. If you get stinky at the gym kewl, if you come in stinky then workout not kewl. People get smelly when they workout, just try and show up clean. Also don't over do it on the perfume or body spray.
Respect, this is really the only rule people have to understand in the gym and life. We are all in it together and respect is vital!