One exclamation point (!) per written piece. Think of it like a diet (you know, one Reese's peanut butter cup/day mentality).
I'm not sure if this is a tidbit or another of my writing pet peeves, but you must consider eliminating the compulsion to put an exclamtion point at the end of your sentences. Let the reader be moved by your words, not manipulated by your punctuation.
When should you use one:
1) Interjections. Used to express an isolated emotion on the part of the speaker ( "Yikes!")
2) Commands. Used to express the weight behind the words ("Get out!")
So take a hint from your keyboard as you knock out your exclamation points: You only get one.
~Cindy Blomquist, WOTH Editor