My writers' group goes round and round about punctuation. Seriously, I think it might be the bane of my existence within the group. (That and speaker tags, but I'll save my rantings on that for another post--or three.) So let me indoctrinate you on Kathryn's Punctuation Style Guide.
Comma: Indicates and interruption or change in phrasing to prevent ambiguity. This is a real hot-button issue among writers, especially when even Oxford University's PR department doesn't use what is known as the Oxford (or Serial) Comma. Some people swear by using a comma before an "and" while others insist it's superfluous. I'm not saying anyone is right or wrong, but there should be a balance in the usage of commas in general. Every time you use a comma, it's like saying to your reader, "Look here! This is important!" So if you're using them in a list or for dramatic pauses, pretty soon they go from clarifying choices to pacing nightmares. But wherever you fall on this issue, make sure you use consistency with your format.
Semicolon: Connects two related ideas, often used in place of a period or conjunction but also with a listing where a comma is not a clear enough indicator. Semicolons are becoming an antiquated form of a conjunction as they are awkward to read and are often misused. In fact, I was reading a book the other day that used a semicolon so well I had to read the sentence twice just to reassure myself that people still know how to use them.
Question Mark: Used instead of a period in cases of an interrogative. Because all questions in writing are rhetorical, interrogatives should be used sparingly and only as a stylistic choice for emphasis or to establish an informal voice. Most languages don't even have variations on a period but instead use verbal indicators to punctuate a question, such as ka in Japanese.
Exclamation Mark: Used instead of a period in cases of an exclamatory remark. Because word choice should carry the power in writing, exclamations should be used sparingly or not at all--choose a better verb instead.
Double-Spaced Period: There should be a single space after a period. Always. This has been the commonly accepted formatting principle for the past 10 years when workers trained to type on computers became more common than those attached to typewriters. If you're still hitting the space bar twice after a period, you're showing your age. Either brake the habit or use "Find" and "Replace".
The moral of this story: Don't waste your ink.