I used to think I was the kind of writer who could easily juggle multiple fiction projects at once. I believed that having ideas spilling out my ears meant that I could and should be writing all of them at the same time. What this really meant, though, was that I never pushed myself through the difficult part of whatever I was writing - whenever I hit the slightest snag, I jumped to another project. I have an unbelievable number of ten-page starts to novels from that period (2000-2006, give or take a few months), and some that made it to fifty or a hundred pages before I ditched them.
I've finally realized that I am a monogamist when it comes to novels. I think the last straw was this November, when I was visiting a NaNoWriMo class at Stanford. I mentioned that I kept trying to call my NaNo main character by the wrong name (the name of The Novel's main character), and one of the professors nodded and said "You feel like you're cheating on your novel." I hadn't thought of it that way before, but it's quite true. My previous approach was akin to someone who, after his/her first argument with a significant other, goes out and starts dating someone else. Immediately.
Beyond the unpleasant simile, I can't bring the same level of focus to a project if, in the back of my mind, I'm allowing myself to run with other ideas at the same time. How do I avoid it? I keep a single document (currently labeled "muddley", used to be called "bits and pieces" or "dumb stuff"), and when I get an idea, I dump it in there. I'm allowed to elaborate up to one page's worth, and then that's it, and I close the document. It's currently 62 pages long. Honestly, I've never taken something out of that document. If an idea has a strong enough grasp on my mind, it lurks there until I finish my current project, and then it demands attention.
Of course, sometimes I have to step back from a novel - say, if I've just finished a new version and need to let it sit before revising. It does feel unfaithful, moving straight to another project, but there's no way around it. Now that I've gotten well and truly into the habit of writing every day, I'm not about to let myself break that habit.
What about all of you out there? Do you favor monogamy or polygamy (polyandry?) when it comes to writing, or are you more of a casual dater? I think I've officially broken the simile.