1. Don't quit when you're losing. Don't say "this is stupid" or complain about your "bad luck" all the time. Hey guess what, board games are probabilistic and have high variance. Usually when you win or lose it's luck, the skill shows up in how often you win or lose. Don't pout and make it obvious you can't wait for it to be over, stay interested in what the other people are doing.
2. Don't drag out runaways when you're winning. When it's obvious you will win and the loser has no hope, it's polite to offer them resignation rather than dragging it out.
3. Always be trying to win. Be competitive. Games are no fun for your partner if you're not trying. Also, it's no fun for everyone if you play your own "meta game" and make up your own goal (like defeating some other particular player in a multiplayer game, or getting the lowest score possible, etc.).
4. Don't make deals or alliances outside the spirit of the game. Also deals and alliances should only be made that are in your best interest, not to intentionally screw someone else (unless that someone else is winning and it's in your best interest to screw them).
5. Don't exploit badly broken rule systems or loop-holes. Try to play within the spirit of the game. It's polite to say "I think I could do this move, but it's totally not fair and breaks the game, so I'm not going to". If you're aware of a rule that others aren't, don't keep it secret and spring it on them, warn them about it. If someone is obviously making a move because they don't know the rules well enough, tell them about it. In general, do not treat knowledge of the minutia of the rules as an aspect of skill and a way to win.
6. Tally your own accounting. Be aware of when you need to pay or collect resources and speak up and do it yourself. Don't rely on others to see it for you. It's good ethics to alert someone a few times, but after that you are under no obligation to do so and should just keep playing fast if they consistently fail to pay attention.