Rules of Thumb

Here's a cool site: Rulesofthumb.org
It's a great reference site for any kind of rule of thumb that exists.
Some examples:
WORK RATING
You can have it cheap, fast, or good. Pick any two.

MARKETING TO THE ELDERLY
For marketing purposes, elderly consumers think they are 15 years younger than they actually are.

BUYING AN APPLIANCE
Any gadget that does many things will do none of them as well as a single-purpose device.