I have a lot of fights with commas. And how I usually win comes down to common sense--how does that particular sentence read, sound, portray what I am trying to say.
I uses as few commas as necessary because I find that the liberal use of commas can distract the reader from the story.
Comma usage is often a question of personal writing style. Some writer use them liberally, while others use them sparingly. Modern North American style guides now recommend using fewer commas rather than more.
For instance, the use of a comma before the "and" in a series is usually optional, but most writer choose to eliminate it as long as there is no chance of misreading the sentence:
Before going home, we went around the mountain, played with the dragon, swam in the lake, and ate our cookies. (comma needed for clarity before "and" if you ate your cookies after you swam in the lake).
Before going home, we went around the mountain, played with the dragon, swam in the lake and ate our cookies. (no comma needed before the"and" if you ate your cookies while you were in the lake).
Do not use a comma to set off words and phrases (especially introductory ones) that are only slightly parenthetical:
Wrong: After dinner, we all played dragon tag.
Right: After dinner we all played dragon tag.
Do not use commas to set off restrictive elements. A restrictive elements is part of the sentence that is needed to make its meaning clear.
Wrong: The dragon's ear, on the left side of his head, was curly.
Right: The dragon's ear on the left side of his head was curly.
Do use commas to set off non-restrictive elements. A non-restrictive element is part of the sentence that is interrupting material that adds extra information---the sentence does not need this element to make sense.
Example: The faeries of Dalorme, who for centuries had lived in caves with dirt floors and rock walls, were unfamiliar with the protocol of living in a palace.
You can also use two other punctuation marks to set off non-restrictive elements: parentheses and em-dashes.
Enclosing a non-restrictive element in parentheses reduces the importance of that information.
Example: The dragons' fire breathing skills (with one exception) were not good.
Enclosing a non-restrictive element within em-dashes has the opposite effect: it emphasizes the material.
Example: The dragons' fire breathing skills---with one exception---were not good.
An what about those very troublesome words: however, therefore, and indeed.
Commas—sometimes paired with semicolons—are traditionally used to set off adverbs such as however, therefore, and indeed. When the adverb is essential to the meaning of the clause, or if no pause is intended or desired, commas are not needed.
Example: A truly efficient fire-breathing dragon remains, however, a lost dream.
Example: Indeed, not one dragon accurately produced the technical elements required in the fire-breathing contest.
Example: If you cheat and are therefore disqualified, you may also risk losing your dragon scholarship.
Example: That was indeed the outcome of the dragon fire-breathing contest.
If you, also, use the word also or too, you, too, should offset those words in the middle of a sentence. The Chicago Manual of Style prefers not using a comma with too at the end of a sentence.
A great place to freshen up your grammar at Grammar Girl:
And sincerely, good luck with your comma woes.