There are few basic comma patterns that show up frequently in most kinds of writing.  Mastering only a few of these will rules will limit comma problems considerably.  More information on any of these rules can be found in any up to date writing handbook.  Look at the following examples:

Comma with a conjunction

Look for this pattern               Complete sentence     , CONJ.            Complete Sentence    

In this pattern, there are two complete sentences joined by a coordinating conjunction like and, but, so or

            Bob went to the crack house, and then he went to the museum of modern art.

The controversy surrounding human cloning is intense, but it is necessary in order to reach a consensus on the issue.

Watch out for places where there is a coordinating conjunction, but NOT two sentences:

Bob went to the crack house and the museum of modern art.

Comma with an introductory element

Look for this pattern               intro. element,            main sentence                       

In this pattern, any introductory element that comes before the main sentence needs to be set off with a comma

When I was a boy, I often walked to school in snow that was so deep I had to use snowshoes.

Yesterday, the guy with herpes simplex 15 was cured by the great scientist.

Sometimes, I like to eat goat cheese in the morning.

Despite the repeated attacks of ten small, knife-wielding boys, I was able to escape to the safety of the cantina.

Commas to set off non-restrictive elements

This one is kind of tricky.  Basically, if the sentence is still grammatical after the phrase is removed, then it needs to be set off with commas.  If the information is essential to understanding the sentence, then it does not need commas.  As a general rule of thumb, most non-restrictive elements use which.

My motorcycle, which is thirty-seven years old and covered in rust, still runs like a wild animal that has been kept too long.

The mayor of Springfield, who likes to call people by disparaging nicknames, was elected last year in a landslide.

These examples DON’T need commas

The corporation that sold me the faulty pogo sticks needs to get some better customer service!

The house that was built by Frank Lloyd Wright sold for $250,000.

Commas with any phrases that break the flow of the sentence

This one is also a bit tricky, but it basically covers any string of words that appears in the middle of a sentence, including parenthetical statements, forms of address, and transition words.

            The Greeks, however, invented democracy in about 1600 B.C.E.

            My proctologist, Dr. Jellyfingers, is one of the best “glove men” in the country.

            Richard, as far as I know, does not go dwarf tossing on a regular basis.