I have a few brief thoughts on punctuation.
The Period. As William Zinsser said, most people don't reach the period soon enough. Don't think of the period as something to tack on at the end of a thought, but rather a tool to kill a winding sentence. There's no rule that dictates the length of a sentence, and while you should strive for variety, shorter is usually better. A long sentence allows for loss of focus, punctuation, and control. Short sentences are good.
The Question Mark. This mark causes few problems. If dialogue ends with a question mark, and the sentence continues, the sentence should follow in lower case: "How are you?" she said. This also holds true for exclamation points.
The Exclamation Point. Writers overuse it. Just look at any advertisement in a magazine, newspaper, or billboard. The exclamation point reveals a gushy author, too excited by his own writing. Instead of forcing excitement at the end of a sentence, rephrase the sentence to emphasize what you want (not the punctuation mark), and let the readers understand the excitement themselves. The same applies to humor—an exclamation point kills the joke, ruins the surprise, and forces itself onto the reader. If the sentence ain't funny, I ain't laughing, no matter how many exclamation points you tack on. Use this mark in these two situations: dialogue that reflects shouting, and exclamations. Alas! That is it.
The Semicolon. Perhaps the least understood and most judged punctuation mark. The semicolon splits a thought, if you will, in two. Both sides of the semicolon relate to each other; both sides must be complete sentences. When used correctly, the semicolon offers a longer pause than a dash, but a shorter pause than a period. When used incorrectly, it stands naked on the page, screaming for help. Avoid using the semicolon often, though don't shy from using it in lists where items contain commas.
The Colon. The colon works to introduce a list. We visited five countries: Germany, France, Italy, Spain, and Algeria.
The Dash. For some reason academia and haughty writers treat the dash as though it were improper and too colloquial. Don't let this thought ruin your use of the dash, which stands as a valid and effective form of punctuation. Use the dash for two purposes: to justify a statement and to form a parenthetical. The parenthetical—you could use parentheses as well—needs to have both beginning and ending dashes, unless it ends a sentence—this being an example.
Note: don't let me catch you appending sentences with two punctuation marks. How'd he do that!? is disgusting. Pick the right mark, and kill the other.