If you're trailering your boat, remember that the person whose truck and trailer is first onto the ramp is the first one to take out, even if that boat wasn't the first one at the ramp.
If you're waiting for a public dock in your 20' along with someone in a 36', and a large or double space becomes available, it's polite to let the larger boat take it, even if you were first.
Always snap your ladder up, and bring in your fenders, before getting underway. Otherwise people will KNOW you're a noob
Don't anchor in a channel. (That's between those solid red and black markers.)
Don't let your anchor line cross another boat's, when in close quarters.
Keep a second anchor on board for the stern when anchoring in close quarters.
Remember, just like driving on the road - you may have the right of way, but don't expect the other guy to yield it properly. Always be ready for him to do a Captain Bonehead move.
Boating Magazine has some great stuff. Also, they have a Facebook page, if you're into that.