Authors have a tendency to construct laws and rules of writing, both to help themselves while writing and to share with others. Here are a few of my favorites, ranging from the serious to the silly:

Frederick Pohl’s Basic Maxim #1:

“Writers write mostly for the fun of it. Agents exist to see they get money for having fun.”

Sir Arthur C. Clarke’s Three Laws

When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
Any sufficiently advanced technology is indistinguishable from magic.

A few of Larry Niven’s (Many) Laws

Never throw shit at an armed man.
Never stand next to someone who is throwing shit at an armed man.
There is no cause so right that one cannot find a fool following it.

Brandon Sanderson’s Laws of Magic

An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.
Limitations > Powers
Expand what you already have before you add something new.

Sturgeon’s Law

Ninety percent of everything is crud.

Scalzi’s Law

Any conversation on the internet will eventually include bacon in some way. And then be forwarded immediately to John Scalzi.

For a big, huge list of eponymous laws, here’s the wikipedia page.