Here are three (although there are probably more) myths:
- Long sentences need a comma. A really long sentence may be perfectly correct without commas. The length of a sentence does not determine whether you need a comma.
- You should add a comma wherever you pause. Where you pause or breathe in a sentence does not reliably indicate where a comma belongs. Different readers pause or breathe in different places.
- Commas are so mysterious that it's impossible to figure out where they belong! Some rules are flexible, but most of the time, commas belong in very predictable places.
Here are some comma rules if we have to have rules. Personally, I don't like rules--I am a writer and not a grammar lieutenant. I write what I write the best way I can write and then let an editor figure out if it needs more or less commas.
Another interesting site is listed on the side bar of this blog. Check it out. In the between time, write some words.