I think more than one or maybe two projects going at the same time is too distracting and this is what causes these projects to become UFO's. I normally work on one thing at a time. I can focus on what I thinking about trying/doing next. If I have too many "things" laying out to be worked on, I spend the majority of my time jumping from one to another and therefore, get nothing done.
The only reason I start something new before finishing a current project is because I have taken a class. When I take classes it is because I want to learn a technique. If I do not continue with the class project, to make a completed quilt, that does not qualify as a UFO. It is learning the technique.
If the technique that I have learned is something I want to continue, that is when I have two projects going at the same time and I need to finish the first project before I continue with the new. This keeps me from building up UFO's. And I have the satisfaction of completion.
I do not have what I consider UFO's. I have many, many quilt tops to quilt. The majority of these are vintage that I have bought or "acquired". I do not consider anything that I have purchased, but not started yet a UFO.
Keep working on one thing at a time and you will be happy.