I enjoyed this article by Ben Halpern, plus all the associated discussion. I always find myself trying to hold on to a project that just sort of slips away as my passions change and new work crops up, both personal and professional. You can dig around in this blog to find examples where I couldn't stick to my ideas for a Unity app and more (where I drifted away from geocaching for instance, although you can't quite grasp how much I shifted toward playing Ingress).
My favorite advice is constraints. I've often found that to be the best way to keep myself engaged. Knowing exactly who's involved (just me?), and a firm, but flexible, deadline (yep, both), and what I'm willing to spend personally, and what my expectations are...and they better be tight, manageable, and focused. As soon as the constraints are loosened and I think "maybe this is something that should be bigger" it goes all cattywampus and drifts away. I was impressed at Minndemo 25 with the devs who had spent three years working on a pet project. That's amazing focus, particularly as they have working software to demo after that time for a crowd of 600+ Seems like a good goal - demo at Minnedemo 28 for instance - but that's a lot of pressure.