How to comment code

Way too often do I see code that is poorly commented. Here are some guidelines that I try to follow for comments:

• Only describe how when complicated
• Always describe what
  • Do this at the file/module, class, and function/method level
  • You can avoid doing it for functions/methods when they're very short or obvious
• Describe why if not obvious
  • Which is most of the time
  • More important than what and how since you can eventually figure those out from reading the code
• Use a style guide that covers comments and refer to it often

I'd love to hear suggestions for improving these guidelines!