I remember reading this quote from someone before, which goes something along the lines of "the best image is the one I'm going to make tomorrow."
Indeed, this is my outlook on photography as well. It is the hope that tomorrow's image will be better than today's that keeps me going each day. It is that hope and anticipation of something more, the spirit of outdoing myself, that motivates me to keep doing what I do. For the day when I say "I've got my best image" is the day I've stopped trying harder.