For me the two are kind of related. I have been working on a side project for the last two or three months. Got going really well for a couple of weeks, and coded quite a lot. Then I got busy with other stuff in my life, so had to leave it for a while. It seems a lot more difficult to get into the state when I haven't been there for a while, while during the good two weeks, I came home from work and managed to get into the state very easily. Enthusiasm for an interesting problem also seems to help me.