[KJUmp]

You have received a ton of good solid advice from everyone..esp. from JRut and Durham. There's really an art to it...understanding when you have to do it and, probably more important, when you shouldn't do it. We don't have a the luxury in baseball of controlling bad behavior by coaches (and players for that matter) through penalties like football and basketball officials do. But that's what I think makes baseball special. We have the unwritten codes. You don't show-up the umps, conversely umps shouldn't show-up the coach or players; while it's in the book that you don't argue balls&strikes..as several of the posters have pointed out...it's not an absolute. You need to set your own level of what you're going to listen to or tolerate. When we decide enough is enough they're done. It's not 15 yards or two foul shots and the other team get the ball.
The thing is though, you'll never develop a rep as a strong umpire if you let whining and *****ing about balls&strikes. You can't let coaches try to manipulate or intimidate you. As JRut pointed out to you...a lot can be said between a coach&ump of the diamond that nobody hears. So as many of the guys have said...communicate. Do it professionally..in a conversational, matter of fact, non confrontational manner. Many times it just nips things in the bud.
Years ago I read a piece on MLB ump Bruce Fromming (as big a red *** as there was when he broke in) and what he said about arguments was this..."there's a right way and a wrong way for a coach/manager/player to argue. If they come at me they better do it the right way or they're gone."
Be confident, look and act professional, be approachable, work on being a communicator, but don't ever be afraid to toss somebody when it's warranted.
Good luck with your career...and welcome to the club.