Placement of comma before quotation mark

In American literature and university courses, the method for introducing a quotation is the same as the British English style: the comma is placed after the last word that introduces the quotation.

Martin said, “Who brought that dog?”

As for the closing quotation mark, American English prefers the comma to be placed before the ending quotation mark.

In her notebook, Sheila wrote that the sun was “beet-red,” but after the game she had seen it “disappear over the bleachers.”

The above mentioned rules are often broken in the context of computer programming. It does not always result from the fact that a poster doesn't know the rules, but it would just be misleading, eg.

After that you need to issue "find . -name *.jpg {};."

In the above example the last full stop is NOT a part of the command and, if typed as such, would lead to errors.

The answers here are correct, if the original question is concerned exclusively with full quotes — aka grammatically whole sentences. If, however, we were to also consider partial quotes (a fragment of a sentence that could not stand alone), the punctuation rules are quite different. In almost every case, no comma will precede a partial quote. If that partial quote falls at the end of the sentence, then the period will of course go inside the closing quote mark.

A partial quote has a variety of grammatical functions in a sentence and cannot get the same punctuation every time.

Larry Trask dealt with this thoroughly in the 'Penguin Guide to Punctutation'. The text is now available online.