I have been working out in gyms for over 25 years. One of my pet peeves is the lack of "etiquette" demonstrated by others who also work out.
On the exercise floor.
First, remember you are not the only one who works out at this location. If there are a limited number of treadmills, elliptical trainers, steppers etc. and many people waiting do not exceed the maximum use time. (Usually 30 minutes).
When you are done sweating on your cardio or weight machine, wipe it down. No one wants to follow behind you and wade through your pool of sweat. Many gyms provide moist towels or sprays and paper towels to leave the equipment as nice as it was when you first got to it. Better yet, bring your own towel and wipe as you work out.
No one wants to be overwhelmed with odor when they have increased their respiratory rate. Women and men keep the cologne and perfume off but please do wear some kind of deodorant. And you know the ones I am talking about they just walk by and you smelled them before they passed you.
Use one piece of equipment at a time. Just because you want to superset your workout, does not mean you get to use all the pieces of equipment at once. If you want to do bench-press and incline flyes, first do one and then the other. Do not take up both benches just for your convenience. If you have numerous sets to do, allow others to work in. Perhaps two of you can trade off between bench and incline presses.
When you are done with your exercise, put the weights back where they belong. There is nothing more irritating than having to pull eight 45-pound weights off the incline leg press machine. Guys, you especially in coed gyms, consider there might be a sixty-year-old woman following you and she will not want to or be able to remove the stack you left behind. Put the dumbbells back in the rack and take the plates off the equipment and put them on the stand when you are done. There are even labels for each of the weights so you can tell where they go.
When you are at the gym, workout! Keep your cell phone in your locker to avoid distracting everyone else. No one wants to hear your conversation when we are trying to get into the groove of exercise.
It is okay if you have a buddy to work out with, just go ahead and workout. Those who linger and chitchat on the equipment while others are waiting are making the exercise experience time consuming and frustrating.
Think about what you are wearing. Yes, ladies, the midriff and cleavage exposing tops are very attractive and you look great, but are distracting. And guys, the over-sized tank tops that show everything and cover nothing too look nice, but also are distracting. Same goes for the baggy shorts that hang down below your boxers - not a good idea for the gym. One of these days they will get tugged off, either by accident or on purpose. And for that matter, do not ogle or stare at someone who happens to be wearing something a little revealing.
If you bring it in, take it out. What I mean by that is, if you have a water bottle, energy drink, power bar, etc. take the empty with you to the trashcan when you are finished. If you bring in an Ipod or other electronic music device, keep it with you. Do not put it on one bench to "hold" your place while you use another piece of equipment.
In the locker room
As on the exercise floor, keep your private parts to yourself. No walking around the locker room showing off your birthday suit. Wrap a towel that fits around you when going from the shower to the changing area. Keep your shower time down as others are waiting to use it as well.
Keep your bathroom supplies in a small area. Remember, other people want to use the counter space or the bench. Do not spread all your hair paraphernalia across the sink as you do at home. Others may need to plug in hair dryers or other equipment.
Do not linger in the locker room. You can have a conversation with your friend outside of the locker room, not seated on the bench where others are trying to change.
In the pool, sauna and Jacuzzi area
Swimming pool lap lanes often get crowded and establishing sides with other swimmers is important. Do not just jump in and figure you can move on it. Speak with the other swimmers to see if you can work in.
Saunas have the potential for some of the worst behavior. Make sure here too you keep yourself covered and keep your eyes to yourself. Follow the rules for the equipment as to whether or not you can add water for more steam. Some saunas are preset while others allow the users to add water.
Go to the gym, have a good workout but be considerate of the others who pay their dues as well.