Commas should be used to make text more clear and understandable, but they tend to be overused. Rule of thumb: when in doubt, leave it out!
In general, if two or more adjectives preceding a noun can be joined with “and,” separate them with commas, unless the noun and adjective are considered to be a unit, e.g., “bad boy.” Use judgment. Too many commas can make writing choppy.
She made a donation to a new political organization.
It will be a frigid, expensive winter.
After city and after state in running text:
The College is located in Middlebury, Vermont, near Lake Champlain.
To separate two sentences connected with a coordinating conjunction, and, but, or—two subjects, two verbs that could be made into two sentences.
Correct: The professor is highly talented, and he will surprise you with his ideas.
Correct: Johnson is highly talented, but Truman isn’t.
Wrong: Jones went home, and unlocked the doors. (Just one subject, no comma needed)
Before and after the year, in full dates within sentences:
The president was born on August 9, 1950, in a New York checker cab.
Between day and year in full dates:
May 1, 2002
After introductory elements that can be eliminated without changing the meaning of the main clause:
By the time you get this message, you will probably have forgotten our conversation.
If you agree with our decision, please sign and return the contract.
It is not necessary to use the comma after short introductory elements, unless needed for clarity:
Before lunch we usually work out.
Separating each item in a series, including the last item:
He brought bread, potatoes, green beans, and butterscotch