Another handy trick: if you don't want to be able to remember a password, use something like a UUID (For example: 4c9837d0-4860-11e1-b86c-0847203c9a66)
Write it down on a random page of a random book.
You can legitimately claim that you don't know the password in this case; it's long enough that you would have a very difficult time even trying to remember it. If you need to be sure, and have the time, destroy said page of said book - now the data is truly unrecoverable by anyone.
Of course, having it in your head is better. In theory, no one can get to that. But, this is useful in limited cases where you might want to deny even yourself the ability to get into something fairly easily.