Just yesterday I was talking about this with my LQS owner. She has many projects going at once. We finally figured out that some projects you just need a break from, and thus another gets started. Or a baby in the family is born, and now a new project takes priority, or you are working on one, and need that perfect fabric for the borders and it gets set aside... etc etc.
I have many UFO's, and what I do is just pick one, do it, and mark it off the list. I have a few projects right now that are on priority, and others that I will get to when I can. I have a couple of UFO's that are more intricate than some of the other projects. Sometimes "intricate" isn't what I can do, so I go with something simpler.
I do admire those who can do one project at a time, but totally accept the fact I'm not one of them, and just go with the flow when the urge and mood strikes to finish off some UFO's. I just pick a project, finish it, and don't forget to post pics!