Whenever I reach a running goal or just feel like I'm stuck in a running rut, I like to give myself a little treat to renew my motivation. Sometimes it's something as simple as a new pair of running socks. It's amazing how some new running gear can suddenly boost your motivation to run.
If you don't regularly reward yourself for running, I'd suggest you give it a try. Just don't use food as a reward, especially if you're trying to lose weight by running. Buy yourself a new pair of running shorts or personalized running gift, or go for a relaxing sports massage or pedicure. Non-food rewards have a more lasting positive effect than food rewards because you won't have those feelings of guilt after you've indulged.