Leave Face Up Cards Alone:
A player who is fidgety or nervous may feel inclined to rotate, flip, and/or manipulate their cards. While this might be acceptable in informal settings where physical manipulation of cards isn't a concern (e.g., pulling out an ace when playing poker), it's not well received in brick-and-mortar venues that have dealt all the face-up cards beforehand so they cannot be disturbed by such actions even if done unwittingly.
Don't Take Too Long:
Waiting too long to make a move in your favorite online game can be agonizing! The longer you wait, the more likely it is that someone else will take an action or get lucky and win. If you're playing with other players who are slowing down, don't let them drag their feet for no good reason- just make your next decision as quickly as possible.
Handle Cards with One Hand:
There are a number of different techniques people use to cheat in casinos. Casinos have become increasingly aware and now implement new security measures for their games, such as keeping the cards near the dealer's fingers while dealing them out so that players can't access them with two hands. As most casino tables don't allow more than one hand on top at any time, it is best not to switch between holding your cards with both or just one hand when playing card games like blackjack--stick to using only your dominant arm!
Signal AND Say Your Decision:
The casino's security camera operators are always on the lookout for signals from their colleagues stationed at a table; but, in case of confusion or error, they can also rely on these hand gestures to know what is going on.