Rules of Poker Etiquette is a good read for some of you new players and for some of us old players with bad habits (e.g. Pauly splashing the pot!)
Here is an important rule: "Don't misdeclare your hand, and don't muck your cards until you have seen you are beaten. If you have a pair of sevens, don't joke around and say something like "I have a straight." Another player hearing you might assume you are telling the truth and throw his cards away, and once the cards are in the muck, he can't retrieve them. YOU might be willing to give him the pot, if you are the only two players in the hand, but if there is a third player involved, he is unlikely to give the pot to someone whose cards he hasn't seen. For similar reasons, don't throw your hand away until you have clearly seen that you are beaten."