"Your attitude determines your altitude."
A little Google search tonight linked the quote to a similar statement made by motivational speaker Zig Ziglar, but I am still unsure about its origin. I found it on a Mac e-mail list I was on back in '98. It's been my favorite quote since.
The quote in my sig currently is linked to one of the Macintosh computer-speech voices; I thought it was appropriate considering New England weather for the past few months.