This advice is geared towards people planning to participate in FAWM 2012 (http://fawm.org), but the same principles apply to any creative effort.
Perhaps the biggest thing that would keep people from writing 15 songs in a month is that they're trying to do too good a job. In general, probably the #1 thing that keeps would-be artists paralyzed is the misguided worry about whether their work is "good enough."
It's perfectly understandable that you want to create something good (or even great). But there's a problem: your mind can't be in creating-something-new mode and in worrying-about-quality mode at the same time. At any given moment, you have to choose one or the other.
Many songwriters find that writing song is a slow, tortured process. For them, it takes a long time to write a song! But in fact they're spending only about 10% of that time in productive writing. The other 90% of their time is lost to worrying. This means that you can write up to ten times faster if you're willing to temporarily forget about whether your songs are good or not. You can write up to ten times as many songs in the same length of time.
So plan on writing 15 quick songs in February, and just know that some of them will be better and some will be worse. And that's fine. Your job isn't to make them come out good. Your job, if you choose to do it, is to write 15 songs by February 29.
Trust your creative process. It will always create the best results that it can, without any need for you to micromanage things.