Generally, more commas should be used in writing to insure proper breaks/pauses so that accurate intonations are better communicated and so that sentence structures are clearer. Commas should be used after each item in a series except the last (i.e., someone has a red, blue, green, white, and orange car).

There are instances in which such a consistent comma usage policy would reduce confusion, such as when items in a series have internal conjunctions. For example, “I love ham, tuna, peanut butter and cheese sandwiches.” Without a serial comma placed after peanut butter, the meaning would be ambiguous as to whether the writer intended that he liked a sandwich that included both peanut butter and cheese or to different sandwiches that included each of these ingredients separately.

The tendency should always be to use a comma when in doubt.