Gotta agree with Lazs and others. It's not about banishing slang, but knowing when to employ it and when to use proper language.
I'll say "dude" a lot, do the secret handshake/handsignal thing and use what's equivalent to "rad", "fat" and "cool" and acronyms like DGIT (Dead Guy In Training) and generally sound like a retard when I'm hanging out drinking with other skydivers.
At work, I'll use work language and the particular vocabulary associated with it.
Other than that, when I talk, I do so to communicate a message to a listener. I'd be a fool if I did not do my best in delivering the message in a way that a) is easily understood by the listener and b) leaves the listener with the appropriate impression of me, my intentions and of what has been said.
Gotta know when to do and when to not do.