Commas and Dates

January 1, 2000, was still in the twentieth century.

On Friday, December 17, 1993, we will be going to the party.
On December 18 we will sleep late.

In February 1999 I started my novel.

Serial Commas

Experts don´t agree about this, but when in doubt, it is safer to use the serial comma. ("red, white, and blue" instead of "red, white and blue"). One of the reasons that it´s safer to use this comma is to ensure clarity, especially in technical or legal writing. There was a court case where there was disagreement as to how the money should be divided among 3 people because of the lack of the serial comma. (The will said that "Bob, Betty and Jane" were to split the money. Because of the missing serial comma, there was confusion as to whether Bob got 50% and Betty & Jane split the other 50%, or whether each person got 1/3.) So to be safe, always use it.

Compound Sentences vs. Compound Predicates

Don´t confuse a compound sentence with a compound predicate. A compound sentence has at least two independent clauses, each containing a subject and a predicate.
Susan finished college, and she is looking for a job.

A compound predicate contains one subject and at least two predicates.
Susan finished college and is looking for a job.

A comma is not needed with a compound predicate.

Introductory Adverbs & Phrases, Part I

Commas are not needed after "ordinary introductory adverbs" or "short introductory phrases" that answer such questions as:

WHEN: tomorrow, yesterday, recently, early next week, in the morning, soon, in five years, in 2004

HOW OFTEN: occasionally, often, frequently, once in a while

WHERE: here, in this case, at the meeting

WHY: for that reason, because of this situation

Introductory Requests & Commands

Use a comma after an introductory request or command.

Look, we´ve discussed this before.
You see, I haven´t finished my report.
Please remember, Christmas will be here soon.

Exception: "Please remember that ..."
When "that" is added, "please remember" becomes the main verb and is no longer an introductory element.

Direct Address

When directly addressing someone, the name or title must be set off by commas.

Thank you again, Mr. Smith, for your kind remarks regarding our company.

Introductory Adverbs & Phrases, Part III

Commas are also used after introductory adverbs and phrases when they function as independent comments (such as IN MY OPINION, BY ALL MEANS, OBVIOUSLY, OF COURSE), which express the writer´s attitude toward the meaning of the sentence.

Commas and States/Zip Codes

If you´re writing a full address in the middle of a sentence, complete with zip code, this is the correct format:

Ex: Our address is 12 Brook Lane, Apt. 3, Kansas City, MO 64011.

If you are not providing the zip code, this is the format:

Ex: Our address is 12 Brook Lane, Apt. 3, Kansas City, Missouri.
(If you want to abbreviate the state, it has to be the old formal abbreviation, which in this case would be "Mo.")

Jr., Sr., III, etc.

Although Jr., Sr., etc., are sometimes regarded as parenthetic, they are part of the legal name and should not be set off with commas.

Examples:
John Wilson Jr.
Bob Jones Sr.
James Stowers III

Commas and States

Most people know about the comma between the city and state, but many forget the one after the state if it´s in the middle of a sentence.

Our office in Kansas City, Missouri, is closest to you.

Introductory Adverbs & Phrases, Part II

Commas are used after introductory adverbs and phrases when they function as transitional expressions (such as: WELL, THEREFORE, HOWEVER, FOR EXAMPLE, IN THE FIRST PLACE), which provide a transition in meaning from the previous sentence.

Commas for Special Emphasis

This is determined solely by the writer. If a comma is used before the phrase, it must also be used after the phrase and vice versa (unless the sentence naturally requires one of the commas.)

Ex: We require a letter from your bank, on bank letterhead, stating that you are an authorized signer.

or

Ex: We require a letter from your bank on bank letterhead stating that you are an authorized signer.

Afterthoughts

Commas are used to set off words, phrases, or clauses loosely added on the end of a sentence.

Examples:
Call our lunch order in, please.
That restaurant is expensive, if I remember correctly.
That bus goes downtown, doesn´t it?