It varies over time, and I've moved through a lot of barriers and become motivated to do difficult things. The key, for me, is to look at both motivators and de-motivators. I focus on the motivators by writing affirmations and reading them aloud every day. I heal the demotivators (usually fears) by acknowledging them. There's a great tool for that in The Artists Way called "Blasting Through Blocks."
When I was very ill, I worked strongly on becoming healthy, but was blocked on writing. When I got closer to health, it was easy to write, but hard to go back and do the things that keep me healthy. I kept at both, though, and I've reached a place where I can stay healthy with fun exercise, so, for the first time in years, most everything is pretty fun and easy.
But it took a lot of hard work to get here!