Getting in is not the whole challenge. The way you behave inside matters, both for your own experience and for everyone around you.
Most lounges don't hand you a conduct guide at the door. But there are unwritten norms. Ignoring them makes you that person.
Volume
Keep phone calls brief or take them somewhere private. Speakerphone in a quiet lounge draws unfriendly attention fast. Most lounges have phone booths or quiet corners for a reason. Use them.
Food and Drink
The food is there for you to enjoy. Not to pack into your carry-on for the flight. Most staff will let it slide up to a point, but it's not the intent. Keep it reasonable.
Same with alcohol. The open bar is a perk, not an invitation. Especially on a 7 a.m. departure.
Space
Don't hold seats. If you're traveling solo, you get one seat. The bag-on-the-chair-next-to-you move gets old fast when lounges are crowded. When it's quiet, spread out a bit. When it's busy, read the room.
Showers
Most lounges with showers have a time limit, usually 30–45 minutes. Use your time, clean up after yourself, and don't linger. There's usually a waitlist.
Dress Code
Most US lounges don't have formal dress codes. Internationally, especially in airline-specific lounges in Asia and the Middle East, smart casual is the safe default. If you'd feel out of place at a decent hotel lobby, you might feel out of place in some lounges too.
Children
Most lounges allow children. That doesn't mean kids get to run around unsupervised. Some lounges have family areas; use them if available. Keep it considerate.