When you are at the gym, and you want to workout on a machine that someone else is currently using, it is considered proper to wait until they finish their set and then to ask them, "How many more sets do you have?" This accomplishes two things: one, it politely lets them know you are waiting and should clue them into asking you if you'd like to work in with them, and two, lets you know that if they only have one more set to do, it likely won't kill you to wait on them. You should not perch over the machine or the person as they do their workout. You should not automatically jump in if they get off between their sets. You should not pace in front of the machine or any silly such thing. Yes, folks, even at the gym, it pays to be polite. The following account is a wonderful counterexample that happened to me today. I was working out with a friend on the hamstring curl machine and he was currently on the machine. We had both done two of our three sets and he was in the middle of his third. An overweight woman approaches who appears to be in her late fifties, early sixties and she looms over the machine and stands so close to me that it sets my personal comfort zone buzzers off.

Ryan: (Looking at woman) We just have one more set each...
Woman: (Interrupting) Well then can I rotate in with you, because you both have been over here doing well over three sets a piece and you're taking too long.
Ryan: (Irritated at her tone) Yes, ma'am. You know, all you have to do is ask.
Woman: (Clearly irritated) I am asking. Don't you know that doing more than two sets of any given exercise is a waste of time?
Ryan: (Flabbergasted) Not really. Different people's bodies are built differently and different kinds of exercising accomplish different goals.
Woman: That's not true. If you read any of the books on the subject, you'll see that more than two sets of any exercise at a time is worthless.
Ryan: (Incredulous) Ok, ma'am...
Woman: (Patting Ryan on the shoulder) You keep believing that, you'll be fine.

This is not a good example on how to behave at the gym. Or anywhere. And when it comes to something like working out, I'll trust someone's advice who actually has worked out for a while over someone whose only exercise in recent memory was flipping the page of the newest fitness guru's book du jour.

-R