One of the most polite things to do when entering a casino is to know and keep the house rules. Each casino will have its own unique house rules (though some casinos will have similar rules nonetheless). If you're new to a casino, it is best that you check the house rules before trying any of the games on the floor. This is also considered correct poker etiquette since you arrive at a casino as a guest.
As you go casino hopping you'll become familiar with some of the house rules that are common among the different casinos. Here is a list of some of the house rules you'll find.
First one would be a thing you do when you walk in: checking your firearm (if you have one) at the door. This is one particular rule you should follow for safety since the second house rule will be related to it: any violation of any casino house rule every casino is given the right and authority to remove a player any time. In other words, the casino has the right to throw you out if they catch you breaking a rule).
Some casinos will have no smoking sections. Funny as it may sound, there are casinos that post "no spitting" and "no farting" signs as house rules. Don't be surprised if the casino suddenly closes your table while you're still playing. Some casinos reserve that right for security reasons.
Another common rule you'll find is to not place anything on the poker table other than the cards, your drink and chips. Casinos love to keep things tidy, which includes care for the cards. Don't bend, dent, or mark them in any way. That house rule also goes for the poker chips. Remember to keep your cards and your chips in plain view at all times. Another common rule is never to touch another's player's card or another player's chips (you might be suspected of theft by doing so with the other player's chips).
Some casinos would prohibit players from using a phone while in a game. Obviously, we're avoiding any chance of cheating. Another common rule is "one player one hand", meaning that you can play one table at a time. Another common rule is avoiding pot splashing, place your chips politely on the table.
Speaking of being polite, remember to be kind to the dealer and to the other players in the game with you. And when you do talk to someone do it in English (or Spanish or whatever language is used in the casino). It is pretty humiliating to get thrown out for being rude at a table.
So, next time you visit a new casino, remember to check the house rules to avoid any trouble. This is in keeping with poker etiquette.