I have one "major" project going at a time but will set it aside occasionally and work on something like a small artsy quilt or one involving a new technique. I got started with Lyn Brown's mini calendar quilts last year and have made most of them (or some variation using my own designs). I enjoy small pieces like this because I can "play" and use up scraps without committing myself to a lot of time and cost. They are also fun to display on their plexiglas stands.
I also make purses and other stuff as a "break" from the major project.
Finally, I've noticed that I have to take a vacation from quilting occasionally and put everything "quilting" out of sight and concentrate on one of my other hobbies for awhile. This will happen when I begin telling myself that I "have" to work on a project instead of thinking that I "want" to work on that project.