What are some "rules of thumb" that you've found to be mostly true?

Rule of Thumb: a broadly accurate guide or principle, based on experience or practice rather than theory.

Some that I’ve heard, that I haven’t bothered to verify myself, is that the higher a button up shirt is buttoned, the more intelligent the person is.

Another is that, in general, you can pack 3 (or 4—I forget) pages into an envelope and not have to pay extra postage.