For my particular group, I'm blunt. Blunt as all heck. (And I often throw in the phrase "pie-hole" for emphasis.) But we're all friends and know each other pretty well, so that helps a lot.
I make sure that I *always* give justification for my decisions, and clearly explain them to the players. I also listen to their responses, and ask
them to justify and explain why they think something should be allowed or disallowed. I've discovered forcing the players to do all the explaining and work saves me a lot of pain right from the get-go (as they often find their justification ends up sounding like "cause I get l33t kewl skillz", and they immediate quit asking once they realize what they're saying).
(Examples of justification includes everything from "I don't own a copy of that book" - this is my policy made clear long long ago; I, as DM,
must own my own personal copy of whatever book is allowed in the game - to "questionable balance and clearly insufficient playtesting", to "it doesn't fit the world that we've been playing for, what, 11 years? Haven't you figured that out yet, monkey-boy? Shut your pie-hole!"
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
)