There's a good article by Lorie Marrero on the Lifehack blog about password security. Most of us have way too many passwords to remember, so we resort to some kind of list to keep track. It may be a written list, a MS Word document, or an XL spreadsheet. Of course, if we lose the list we're in deep trouble. If someone else finds it, we're in even more trouble.
Another alternative is to use the same password for everything, which is an ivitation to disaster. If someone gets their hands on your one-and-only security code, they can gain access to just about your entire life. Another point Lorie makes, and one I've never thought of myself, is what happens if you become incapacitated. How much trouble would it cause if no one knew how to access your stuff?
There are a number of software tools that you can use to protect your passwords and another recent Lifehack article listed ten free ones. Some reside on your computer, some store your information online, and some do both.
If you use a paper-based system, Lorie suggests that you not label your password notebook with "PASSWORDS" in big, bold letters. A further suggestion is to use a code system for your written records. For example, if the password is fido1995 you might record it as dog + year. Just make sure you can remember your own code.
In case your idea of a secure password is your phone number or you cat's name, here's another Lifehack article on creating strong passwords. With identity theft a growing, serious problem, password security isn't something you should take lightly.