So here's a few instances where authors tend to go wrong...
- He said, "Thank you, David." There should always be a comma separating the rest of a sentence from the dialog. Also, always have a comma before a name that is neither the subject nor direct object of a sentence.
- As I hung up the phone, I leapt toward the kitchen. A comma should be used when indicating a pause for the reader. In this example, the reader can't run straight from phone to I. They don't connect. They are separate, though related, and must be connected by a comma.
- It’s strange one of my favorite things is sleeping when I’m an insomniac but, then again, maybe not. You can remove the intervening "maybe not" and still have a complete sentence. This is a major one authors mess up on. When you have a portion of the sentence surrounded by commas, test to see how removing it would influence the sentence. If the sentence isn't right, you put the commas in the wrong place. A common example would be putting words like "but" or "and" on the wrong side of the comma. If I put "but" on the other side of the comma, the sentence would read, "It's strange one of my favorite things is sleeping when I'm an insomniac maybe not." That sentence doesn't make sense. It's wrong.
- Eating and sleeping had always been my favorite activities, though. This one is probably a redundancy of one of the previous bullets, but I thought I'd put it in anyway. The sentence is complete without "though". It's an addition to the sentence. A pause in reading is required.
I'd love to show some great examples of improper comma usage, but that would require posting copyrighted content on my blog, which I won't do without permission. The one to pay the most attention to is the third bullet. It has a massive influence on sentence meaning and can leave a reader confused, reading a sentence over and over trying to elucidate what the sentence is supposed to say.
And I was intending to make a graphic for this post, but it's after midnight, I'm exhausted, and I have to get up at 6:30a.m. tomorrow (yay, not...). Wait, make that today. Grrr... I think I'm going to die...