Artemis' post on the usage of the comma is an excellent resource, but if someone wants something can be kept in her/his head, I suggest the SPIT rules for commas.
I used to give this to my students and I found it helped a lot.
SPIT
S = use a comma in a series of words, phrases or sentences. e.g.Clark, Lois, Jimmy and Perry work at the Daily Planet.
P = use a comma to separate prinicple clauses. (A principle clause is basically a sentence that can stand on its own.) e.g. Clark eats junk food, but Lois eats chocolate ice-cream.
I - use a comma after an introductory word, phrase or clause. e.g. As soon as Clark hears a cry for help, he runs off and changes into Superman. However, no one notices him do this.
T = use commas to separate the thought interruption from the rest of the sentence. e.g. Lois Lane, investigative reporter extraordinaire, never recognized that Clark Kent, a mild-mannered reporter, was Superman, a strange visitor from another planet.
Hope that helps.
gerry
