1. Keep your speed down. People are always in a hurry, whether it's getting on your way first thing in the morning or trying to get back home after a long day being on the water. When you're pulling into the marina slow down and be courteous. Not everyone knows what a spring line does and any wake can send their boat rubbing against the dock. It can also be a safety issue. Someone may be pulling out of a slip as you're pulling in. Stopping a boat is nothing like stopping a car and you may end up with two boats in one slip. That gets crowded fast.
2. Clean your dock. I'm not saying you need to get a broom out and start sweeping the marina. Just keep your area tidy. There is no reason to have your lines strewn all over the dock to create a triping hazard. Coil your lines and shore power cords. Also, if you dock bow in, use good stern lines and keep your pulpit from hanging over the dock. We already know you own a boat, we don't need it to become a pulpit measuring contest.
3. Quiet hours. Some marinas have a lot of liveaboards. Adhere to any posted quiet hours. Sound travels on the water and if your 3am party can be heard across the entire bay, you can bet the boat next to you is plotting how to "accidentally" open your seacock as soon as you go to sleep. Keep maintenance to daylight hours if possible. Running engines or beating on things with a hammer gets loud quickly. This also applies to your electronics. If you're away from your boat for a few hours or more turn off your unneeded electronics and secure any wind generators. The VHF chatter and constant whirring of a windmill can be irritating.
4. Trash. Dispose of your trash in the marina trash cans or below deck. If you want to live like a slob, then live like one in private. If you have old food or other garbage outside it will draw all sorts of animals and birds. Some people may like the soothing colors of bird droppings on their deck, but most don't. Don't leave anything out to attract them.
5. Tighten up your lines. Don't leave any slack in any of the sheets or lines on your boat while you're away. This will not only chafe your lines quickly, but the constant clang, clang, clang of a shackle banging on your mast will drive everyone insane.
Remember everyone is down enjoying their boats, stop by say hi. Introduce yourself with a smile and a handshake, instead of introducing yourself as the annoying guy of the marina.