Shannon Appelcline has a good article posted on Boardgamenews.
Our group seems to practice these rules pretty well already, but I thought everyone might enjoy the article.
The Ten Commandments of Gaming
Here's a sample:
1. Thou Shalt Be Even Tempered and Good Natured. Be not surly if the game goes poorly for you, if a neighbor breaks a deal, or if you feel your position is impossible to win. Instead, go forth with a smile, and thus continue to enjoy the game yourself and make it enjoyable for others. Further, address your temperament to the mood of the game. Do not be loud and boisterous if your gaming partners are not, and similarly do not be dour and sullen when there is an air of frivolity upon the table. If the temperament of the table does not suit your own, then find another table rather than forcing your mood upon others.
2. Thou Shalt Not Blame Thy Failings On Others. Neither blame the dice nor the game for your loss. Do not harp upon how badly designed the game is--unless perhaps it is the sentiment of the entire table. And especially, never blame your losses upon the bad play of others. He who sayeth “The winner of Puerto Rico is he who sitteth to the left of the newest player” is an arrogant ass who was not invited back for the next game. If you can, instead speak well of others’ plays, congratulating them upon the brilliant moves which pushed you down into ignoble defeat.