Personally I comment when I think it needs a comment :-)

My commenting style has gone from none (long, long ago when I was young and innocent), to far too much (after the first few times I had to maintain somebody else's code :-), to what is hopefully a happy medium.

I've noticed in recent years that things I would have previously elucidated with a comment I now explain in other ways. For example:

• I write a lot of tests.
• I spend a lot more time and effort on the naming of variables and methods.
• I spend a lot more time refactoring code for clarity and ease of understanding (as opposed to functionality or efficiency).
• I use assertions, preconditions, postconditions and other design by contract techniques.

These have an advantage over textual comments because (apart from naming) they cannot get out of sync with the code base.