Setting mini goals and deadlines based on upcoming events is motivating.
Only run flat areas and uphills to avoid pain.
I weigh once per week and once in a while the scale does not got down. When this happens I take my measurements.
My secret is so strange that I'm a little embarrassed to post it. However, I've been doing this for at least 8 years and the rhythmic chanting helps me still! I happen to LOVE exercise, but chanting (occasionally) helps me to improve my speed to reach my goals.