On good days, I like this one which I've stolen from someone's signature line:
Life should not be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, chocolate in one hand, martini in the other, body thoroughly used up, totally worn out and screaming “WOO HOO! What a ride!
On bad days, I go with John Cougar Mellencamp's "Life goes on long after the thrill of living is gone."