An umpire's worst nightmare is to be overturned on an appeal after the game. Even if it means overruling yourself. DO NOT become best buddies with managers, coaches or players. This has enormous potential perceptual ramifications. Be pleasant and friendly but not overly familiar.
First names are fine.
Going out for a cocktail after the game with the manager is not. These facts get around faster than you would believe. You are a team, and should always appear that way. Also, there is strength is numbers. Walking around while your mask swings from the strap is another bad and silly-looking habit that some umpires have.