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.