I read Don Aslett's book, "Is There Life After Housework." In it he says the best way to be productive and get things done is to have several things going at the same time. So you work on one thing until you're tired of it, then move to the next, to the next, to the next. Then we're thinking fresh the next time we come around to it.
About 12 years ago, I decided to catalog all of my started projects. I had 75, and decide I had a problem! I'm trying to only work on 12 or so at a time.