I have two different modes, and a set of rules for each.
Mode #1: Burning with Inspiration
Sometimes, I get borderline manic with inspiration. It takes me over when it arrives. Sometimes a crush on a new muse or some particularly inspiring new music will set it off. Others, it seems to come out of nowhere. But whenever I'm in this state, I know I'm supposed to create something. I become intensely fixated on that one thing and can't focus on anything else (including my day job, which is a big problem). I spent so many years fighting against these waves of inspired mania when I was younger that I missed out on a lot of work I could have gotten done. When I'm in this state, I can produce so much more than normal.
Recently I've decided that when this happens to me, I need to cast everything aside and focus on whatever my inspiration is showing me to do. It doesn't happen often enough that I need to worry about it seriously messing up my life, so I'm just going with it and doing whatever it wants. If that means dropping my current novel to work on a new project, so be it. This happened to me a few weeks ago, and now I have a 30K novella to play with.
Mode #2: Little/No Inspiration
At some point, that intense inspiration always fades away, and I enter back into my normal state. In my normal state, I can work on whatever I choose to put in front of me, so when this happens, I look at everything I created and decide using logic what to work on next. I do day job work and make $$ to support myself, and at night, I just work on whatever novel I would like to complete first. I keep going with the new plan until another wave of major inspiration hits (there's usually a couple months in between), then the process starts over.