Be stoked on yourself and everyone at the park. Whether it's super hard trick or someones first ollie, be supportive of people making progress. Skateboarding can be frustrating, but stay positive with your progress, and don't throw your board around the park.
The features of a skatepark are meant to be skated; don't block these features. This includes your board, jacket, bag, jambox, even where you're playing a game of SKATE. If you fall (and aren't seriously hurt), hustle out of the way.
Don't intentionally do the trick someone is working on, right in front of them, to show them how easy it is. That's one-upping, and is usually seen as a being a jerk. Especially when repeated. People will want to work on the same trick, it's usually not a big deal. Some people ask about doing the same trick. Just be courteous when others are struggling with a trick.
Don't cover a ledge, curb, rail, or coping in wax if there are other people skating it. If you want to wax something it's usually worth asking, or at least warning everyone. Fresh wax can be dangerous if someone's not expecting it.
A clean park is a fun park. A bag of chips in the middle .. can ruin your line.
All different types of people have heard the good word that is skateboarding, and on a sunny Saturday afternoon they'll all be at the skatepark. Everyone should feel welcome to skate there.
Skateparks can get busy on a nice day (or a rainy day for indoor parks), but everyone deserves to skate. Usually a sequence, or rhythm will form, so find where your spot is in it.
Skatepark is for skating, so while having a good time is strongly encouraged, it's not a party.