When you DO put a comma after "said":
* Direct quotes: "I'm going to the store," said John.
* The comma separates the quote from the speaker's name and verb.
* To avoid confusion: "I saw the movie," said Mary, "and it was amazing."
* The comma separates the first part of the sentence from the continuation of the quote.
When you DON'T put a comma after "said":
* Indirect quotes: John said he was going to the store.
* No comma is needed because the quote is not directly presented.
* When "said" is part of a phrase: John said he was going to the store, but then he changed his mind.
* No comma is needed because "said he was going to the store" acts as a single unit within the sentence.
Remember: It's always a good idea to read your sentence aloud to see if it sounds natural. If it sounds awkward, a comma may be needed!