Study the following examples very carefully.  If you feel a strong urge to place a comma somewhere, you probably have comma trauma:

Use pertinent information whenever possible.

The associate who joined the company most recently is the one who will be laid off first.

All the names that appear on the list are supposed to be typed and printed out.

The product has an infinite shelf life as long as it is kept totally free from moisture.

Late arrival will not be a problem unless the boxes arrive unmarked or the shipper fails to leave a receipt.

If you feel the urge to place a comma somewhere in the above five sentences--if comma trauma is gripping you--then reading the sentences again may be a more useful remedy than studying comma rules.

There are, of course, valid reasons to place commas in sentences.  For example, a comma is definitely needed before and, but, or, nor, for, so, and yet when any of these words is connecting two independent clauses:

I started reading the article this morning, and I finished it after dinner.

She wanted to ride her bicycle to the post office, but she decided to drive instead.

You should also use a comma to exclude nonessential information in a sentence:

I asked her what, if anything, I could do to help with the arrangements.

The last time I shopped there, by the way, was almost a year ago.

You should also use a comma after an introductory clause.

Examples:

As soon as the shipment arrives, we will contact someone in your office.

Because we care about service, someone will be on call seven days a week to assist you.

Commas are extremely useful tools, but their effectiveness is diminished when they're overused.

Forget about placing commas where you believe the reader is likely to pause.  Start thinking of commas as separators rather than pauses.  That way, you won't be as likely to place one where it does not belong.  Above all, remember that there should be a valid reason for each comma you insert.  So if you don't need it, delete it!