Well, I don't think there's anything wrong with letting your side projects die or collect dust. It's perfectly natural to get partway through and then decide you just don't want to do it anymore.

I've found that if I REALLY want to do something, I'll just find time to do it. Unfortunately I haven't found a way to fake that kind of motivation. It's kind of there or it's not.

One technique I've used though, is to make my side-projects bite-sized (little tiny MVPs). That way, even if they only hold my attention for 72 hours, I'll still be able to finish them and put something up live. Later on, if I'm still interested in it, then I can iterate on it more.