What's the comma for? It's to separate out units of meaning in a sentence, so the reader knows what words go with what words. So, for example, an appositive (which is just a phrase that modifies a noun, like "Jenny, an experienced skater, loves the winter") is a unit of meaning, and so goes together, and is separated from the noun and whatever follows in the sentence.
(I am NOT going to get into restrictive vs. nonrestrictive here, because it gives me a headache to explain, and anyway, it's been explained better by others:
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Jenny, an experienced skater, loves the winter.
-- here's what it looks like without the commas setting off the appositive:
Jenny an experienced skater loves the winter. (*This is WRONG.)
Not only does the phrase an experienced skater just blend in without meaning, but the subject and verb (Jenny... loves) have no prominence-- just two more in a series of words.
Both commas are needed: The appositive is a UNIT of meaning and so should be treated as a unit-- comma before and after, not just one or the other comma.
The other most annoying comma mistake is to skip the comma after a direct address. Fiction uses a lot of dialogue, and so you'll have more direct address (using someone's name while speaking to him/her) than in non-fiction. The name, when the speaker is directly being addressed, is set off from the following sentence with a comma.
John, I don't know what you mean.
(If the name is in the middle of the sentence, then there's a comma before and a comma after:
I don't know, John, what you mean.)
That is, the name is a unit of meaning IN THIS SENTENCE. And the "sentence-proper," the main clause, is another unit of meaning.
What if the name is the subject of the sentence, or another part of the sentence? Then probably there's no comma, because it's part of the unit of meaning:
John didn't know what you meant.
You didn't know what John meant.
But here's what counts: If I get a manuscript and see on page 1 an elementary mistake like
Hello Jody what's going on?
I have to wonder how many other elementary mistakes there will be in the pages to come. Not only presumably has the author been exposed to the correct way in school (no, I don't want to hear that "no one teaches grammar anymore"-- I teach English, remember?), but also she's read books with the correct form since she was six or seven. (A good reason not to read unedited books is that they teach bad habits.
The editor has to fix every single one of those. Every one. And in some books, that might be an average of one a paragraph, three or four a page, and that's only ONE chronic error. And-- getting sick of me saying this yet?-- this is the writer's job. Making sentences meaningful to the reader (and that's the purpose of syntax and punctuation) is the writer's job.
Now most of you don't make elementary mistakes. (You probably only make the complicated mistakes that have us consulting handbooks and other editors and waking up in the middle of the night debating options. :) But be ruthless with yourself. Read the grammar handbooks. Study the examples. Compare those against your own manuscript passages. As you read edited books, notice not just how the author develops suspense, but how the author structures and punctuates sentences. Pay attention. Have a friend who is good at editing go over your manuscript-- even two or three pages would help!-- and point out your chronic errors. Don't let the friend fix these, because you won't learn unless you fix these yourself. Most important, challenge yourself to turn in clean manuscripts so that the editor can focus on the plot and characters and not the punctuation.
Make yourself absorb the rules and apply them. Grammar and punctuation are much, much easier to learn than plotting!