Too many exclamation marks can make your prose look like Facebook writing or teen texting. Most editors don't want to see more than one or two in a chapter, although they might allow one a page in a book written for middle grade readers.
It's best to save them for short emotional outbursts like "Holy cow!"
Don't use them for statements of fact no matter how excited the speaker is. Show excitement with word choice and gestures, not with the exclamation mark. Don't let your narrator use them as if the narrator is showing the reader how fascinating or interesting the narrator's remark is supposed to be. The reader can decide whether to find something fascinating or interesting without the narrator's coaxing with an exclamation mark.
And then George stepped in the puddle! <-- Why must the narrator intrude with the exclamation mark to prod the reader into finding this humorous or significant?
And never double punctuate. You don't want to look like advertising copy:
New! Improved!! Get yours today!!!
(This applies to ?! and !?, too. I read many books; I have only seen one book by a bestselling author that contains these. (Dan Brown's Inferno.)
Avoiding exclamation mark overuse isn't just my opinion. Here are what other writing teachers and editors say about exclamation marks.
The exclamation mark is an enthusiastic bit of punctuation that appears at the end of far too many sentences. In their eagerness to show their emotions on the page, many writers carelessly splash these marks about as if there were no price to pay for their use. Yet there is a cost--that the writing will seem immature and unseasoned. …Exclamation marks seem too much like middle school cheerleaders eager to please. They jump and shout with great vigor and feeling, while making the written words appear juvenile. http://www.gaylamills.com/avoid-exclamation-marks.html
The exclamation mark is often the lazy writer’s way of conveying emotion. By using it, you’re telling the reader how to interpret the line. “Pay attention,” you say to the reader, “this is going to be exciting.” Instead of relying on this rascal of punctuation, you should choose your words precisely so that they carry the emphasis themselves. Focus on using vivid and striking nouns and verbs that capture the essence of the strong emotion you want to convey.
H. W. Fowler is quoted as saying about the exclamation point: “An excessive use of exclamation marks is a certain indication of an unpractised writer or of one who wants to add a spurious dash of sensation to something unsensational.”
In Self-Editing for Fiction Writers, Renni Browne and Dave King say:
“Exclamation points should be reserved for moments when a character is physically shouting (or experiencing the mental equivalent). When you use them frequently, you begin to look as if you are trying desperately to infuse your dialogue or narration with an excitement it lacks.”
Strunk and White, in The Elements of Style, write, “Do not attempt to emphasize simple statements by using a mark of exclamation. The exclamation mark is to be reserved for use after true exclamations and commands.”