In some of the early cash games I played, we had one player who would consistently hold his cards below the table, out of sight. It was a group of friends, so the assumption was that he was not cheating (and his consistent losses backed this up), but he had to be told repeatedly each game, and occasionally caused betting to go out of sequence because other players didn't realize he was still in the hand.
I've seen similar behavior from other players, usually those who are new to live games, or poker in general.
I have had similar experiences with players making string bets, or failing to observe chip etiquette.
In most cases, the offenders corrected their actions after the problem was explained to them, but some either felt that it was an arbitrary rule they didn't have to follow, or simply seemed unable to remember to avoid the behavior.
Aside from the obvious step of not inviting these players back, what are the best ways to address this without ruining the current gaming session? I would like to know guidelines for both "friendly" and "serious" games, if they would be handled differently.