Another rule that I doubt anyone will disagree with.
With the rule "Memo Yourself," Masello advises writers to keep a pen and pad (or the equivalent) handy at all times. You never know when an idea will come to mind, and if you fail to jot it down, you will lose it. Later on, when you remember that you had an idea, you will rack your brain trying to recall what it was...and you will be convinced that it was the most brilliant idea you've ever had, gone forever.
I know of no writer who ignores this advice. (Well, Isaac Asimov claimed that he was always able to remember the ideas he got even if he didn't jot them down right away, but he's an exception.) Everyone does it differently. Some writers use a pen and pad, and keep it next to their bed at night, in case they get an idea. Others use a small digital recorder; this is especially handy if you're driving and can't stop to write. One writer I know collects his ideas on paper and then puts them all in one big folder in his computer, which he calls his ldea garden. The incomparable farwing uses a Moleskine notebook. But the key point here is that every writer has a method of jotting down an idea as soon as it comes to mind.
And you have to do it, no matter what. Last night, Nomi and I were having dinner with friends, and an idea came up for a story. I took a moment away from the conversation to jot down the idea down before I lost it.
I don't use pad and paper anymore. My own idea book has evolved. Once I used a small notebook to keep all my ideas in; nowadays, I have a category called "Idea Book" in the Memos program of my handheld PDA. Of course, I keep the notes backed up on my computer; I shudder to think about what would happen if the notes got deleted by accident.
On a further personal note, following this rule is difficult if you avoid writing on the Jewish sabbath (from Friday sundown to Saturday sundown). So whenever I get an idea during that period, I have to rely on my memory to avoid losing it. My solution is to rely on gnomi's memory as well. I find that if we repeat the idea over and over, she tends to recall it after the sabbath is over. (In fact, one my Hugo-nominated stories never would have been written had Nomi not recalled the idea, for which I am grateful.)
So what method do you use to jot down your ideas?