I recently did something like this for the unlock password of my laptop--I type that one fairly often, but if the passphrase isn't super secure (after all, how many bits of entropy in an affirmation? ;-) ) it doesn't matter that much since it's the physical security of the laptop that's most important anyway (I've been considering removing the whole password-to-unlock part, I did not because I like the background I picked for the login screen--now I have a better reason not to).

Currently it's a mantra from a meditation exercise that I particularly liked, but that was just the first thing that came to mind when I realized "hey, I type this multiple times each day, so why not have it be something uplifting, cool, self-affirming or just nice?". But I think I'll change it to something more targeted according to the pointers given in that article as soon as I come up with something good.