Rule #2: Locker selection is key.

If there are ample lockers available, there is no reason to take the one right next to the only occupied one. According to the law of probability as it applies to gym lockers, you and that person will need to get into yours at the exact same moment. This may result in, at the very least, discomfort, and at the very most, serious injury. Plus it's creepy.

Rule #3: There are no mats in the locker room for a reason.

So you want to stretch after (or before) your workout. Good for you! You're doing right by your muscles and joints. You may not know this, but many gyms come equipped with their very own room for that. Certainly there's an area with mats and balance balls. Do you know what this means? There is absolutely no reason why you have to stretch in the locker room. There's no room for that. And if you must ignore the rules and stretch in the locker room, please don't do it nearly or completely naked (see #1 above).

Rule #4: There's a time and a place for making new friends.

People don't generally like to talk to strangers when they're in a compromising position, i.e., in various states of undress in a locker room. If you feel the need to be a Chatty Cathy, take your cues from the person you're trying to engage. Does she avoid eye contact? Seem hurried in her answers? Blush? These behaviors may indicate the person would like you to stop talking to her.