I just ran across this site by Ted Montgomery, with useful pages about punctuation. It’s easy to look things up and get good examples of correct usage.
http://www.tedmontgomery.com/punctuation/index.html
Believe it or not, some readers will judge you on your commas and colons. So it doesn’t hurt to get them right. I wouldn’t lose any sleep over them, but still, when all you’ve got are your words on a computer screen to build credibility, go for perfection.