Go down to the store and get a matching top and bottom set and a new pair of semi-expensive or expensive sneakers. It may be totally 'girley' to say this, but if I like what I see when I look in the mirror to start, then I will keep going at it. The reason I say by somewhat expensive shoes, is because if you don't use them, you'll be wasting money and that in itself can be motivating....
I go to zumba 3x week (one is a toning class) and LOVe it. I get there early and grab some weights from the gym, and go into our aerobics studio and do some curls and stretches with 15lbs weights (few reps each) just for bulking. I'm no she-ra muscle type, but I can throw my fair share of hay bales or 50lbs bags of feed around as a result and I feel better about myself just having that extra strength to do chores.
Sometimes I wear a shirt with a big "S" supergirl emblem on the front when I'm feeling really slow or sluggish and somebody will always say something about it at zumba, and so I find that the social interaction that comes with working out with others feeds me energy, too.
So that's my motivators - cute clothes, a little muscle to help with farm chores and social interaction. Plus I've lost 12lbs so far.