I wouldn't feel comfortable attributing this cliche/poster thingy/etc. to anybody, but you can find attributions if you look around:
"It's not that `today is the first day of the rest of my life,' but that now is all there is of my life."
--Hugh Prather
http://rachelkeller.homestead.com/quottime.html
Today is the first day of the rest of your life.
~C. Dederich
http://www.richmond.edu/~ifarkas/quotations.html
"Today is the first day of the rest of your life!"
Kurt Vonnegut
"Breakfast of Champions"
http://oxygene.ibspan.waw.pl/~sikorski/
If today is the first day of the rest of your life, what the hell was yesterday?
~bumber sticker
http://www.cyberquotations.com/humorous.htm
"Also, get PERSONAL POWER over your POTENTIAL and LEARN HOW TO ASSERT IT!! Grok yourself fully during QUALITY TIME!! Dare to be great! Remember: TODAY IS THE FIRST DAY OF THE REST OF YOUR LIFE!!"
~Kermit Busganglion, The Hand You're Dealt
http://www.trends.net/~roversct/Quotes/principles.html