Anything I've done that really worked happened because, either by sheer will or a lack of options, I was incredibly focused on one problem. Blogger started as a side project of my first company, Pyra Labs.
I'm a Gemini, so I have a butterfly mind. I'm curious about everything, and while this is a fine trait for a writer, it also leads to incomplete projects. Every year. (I have a "Later" folder for each year, dating back to 2004.)
Evan Williams -- of Blogger and Twitter fame -- has great advice.
I'm taking it to heart. Perhaps it will help you, too.