Though they're actually more of guidline than actual rules...
1- "1+1=2" That is, if I shade something once and then shade over it again, it will be twice as dark.
2- "The first cut is the deepest." The above rule does not apply to the very first line or shade, since compared to nothing, something is everything.
3- "The eraser is your friend." Erasing is not a sign of failure or inability, it's simply another tool. Perhaps if we changed its name from the eraser to the 'unpencil.'
4- "Darker is darker, unless it's deeper." Shading does two things: it denotes darker colors and it indicates a greater depth from the viewer's perspective. Black can look like a color or a hole, depending on how it's applied.
5- "Trust the lines." Look at the lines of anything you draw, one at a time. Ignore the big picture most the time, and just copy the lines: they know what they're doing.
6- "Feel before see." Specifically in regard to portraits, look at a person holistically before you analyze their lines. In the world of portraits, it is just as important to have a drawing feel like the person as look like them. It's like every person is a bell, and when you look at them, you can feel their unique tone. For those with whom you have little or no emotional connection, such as complete strangers, the tone is quieter, but it's there. Learn about your subject, take a moment to talk with them, find out their name. It's much easier to put someone on paper once they're not a stranger anymore.