If you want to strongly emphasize a sentence, either because it’s emphatic or humorous, you may sparingly use a exclamation point. But use it sparingly! It’s supposed to express strong emotion. Don’t use more than one at the end of a sentence, unless you’re a strongly-emotional fourteen-year-old girl writing on MySpace.
Much better: use exclamation points no more than once per page. Like the boy who cried Wolf so often that nobody believed him when he saw a real wolf, you don’t want to become known as the writer who cried Exclamation when there wasn’t really anything worth exclaiming about.
If the exclamation is part of a quotation, put the exclamation point inside the quotes. So, if it represents your strong emotion, put the exclamation point outside the quotation marks. If you’re quoting someone else with strong emotions, put the exclamation point inside the quotes.
I can’t believe that you used multiple exclamation points when you wrote, “Whatever!!,” especially at your age, Grandmother!
The same can be said about interrogation marks. One is enough, even if you are really curious about what you are asking. Maeve Maddox once wrote a post here recommending writers to let the word do the work. Well, they should also let the punctuation mark do the work!
No multiple exclamation or interrogation marks, please.