The usual stuff we all read over and over anyway:
.) It's a game to be enjoyed by everyone playing it.
.) Be friendly.
.) Bring all the stuff you need for playing.
.) Don't ever touch anybody's minis without permission.
.) Don't even think of things that could give you an unfair advantage over the other player.
.) Forget everything you find clever and funny in terms of "psychological warfare" "expanded tactics" or whatever you call it. Acting like a dick is NOT part of the game. Annoying/insulting other players is not a strategy, it's antisocial behavior.
.) Try to avoid rules debates. If you can't agree, roll a dice.
.) Rolling dice is a way to get random results. Odds are influenced by the number and kind of dice that are rolled. There is no such thing as "rolling techniques to get you better results" or "learning how to roll a dice". All these things are cheating. (I know, it sounds weird but there are some people who think that you can or should "learn how to roll well").
All in all: Be someone others will want to play a second game with. Playing 40k is about all people involved having a good experience. Looking at 40k objectively, it's not a very good game, it's never been in the least suited for "serious tournament play" and never will be. So winning can't be the only objective.