Motivation comes and goes. It is "commitment" that will carry you through when your motivation wanes.
Be like the little engine that could and know that the day WILL come when you will slip on your diet and make a commitment to get back right on track immediately NOT after the weekend of starting Monday.
But nothing wrong with motivating yourself at the same time though. What I do is I have mini goals and when I reach them I go and treat myself to something I have wanted for a while but I have been too cheap to buy it.
When I lost the first 20 pounds I bought myself some new expensive brand name (I usually get WalMart! LOL) walking shoes.
When I hit 40 pounds I treated myself to a trip to the mountains.
When I reach my goal I am going to buy myself either a new hybrid car or a motorcycle I haven't decided yet but I have plenty of time.
How's that for motivation! LOL! Motivation for material gain ... thats what works for me!