1. Use a comma before a coordinating conjunction (and, but, or, for, nor, yet, so) that joins two independent clauses.
2. If the clauses are short and closely related, a comma is not required.
3. If the coordinate clauses are long or themselves contain commas, you can often avoid confusion by separating them with semicolons.
4. Use a comma to separate an introductory element (clause, phrase, conjunctive adverb, or mild interjection) from the rest of the sentence
5. Use commas to set off parenthetical elements or interrupters (including transitional adverbs)
6. An important distinction must be made here between restrictive and nonrestrictive modifiers. Restrictive modifiers are essential to the meaning of the sentence in that they restrict that meaning to a particular case. Hence, restrictive modifiers are not parenthetical and cannot be removed without seriously damaging the meaning. Since they are necessary to the meaning, restrictive modifiers are not set off by commas.
6. Nonrestrictive modifiers are parenthetical. That is, they digress, amplify, or explain, but are not essential to the meaning of the sentence. These modifiers simply provide additional information for the reader--information which, although it may be interesting, does not restrict the meaning of the sentence and can be removed without changing the sentence's essential meaning
7. Use commas to set off parenthetical elements that retain a close logical relationship to the rest of the sentence. Use dashes or parentheses to set off parenthetical elements whose logical relationship to the rest of the sentence is more remote.
8. Use commas to join items in a series. Exclend in journalism, this includes a comma before the conjunction that links the last item to the rest of the series.
9. Although not called for by any of the above principles, commas are sometimes required to avoid the confusion of mistaken junction.