I'm trying to write one song per day for my February Album Writing Month project. If I can keep up with this, I'll reach the goal of writing 14 songs on 2/14, Valentine's Day, and then I can spend the second half of the month recording quick demos of some of these new songs.
In the past several years, most of my songwriting has been prompted by being struck by inspiration, or has been driven by a fairly specific assignment. It's a little harder to start writing "cold," with nothing to go on. I've turned to my file folder of song ideas collected over the years, and I've also resorted to random brainstorming, just casting about for any idea that catches me. So far I haven't found myself stuck with nothing to write about, but we'll see if the idea stream holds up for 11 more songs.
I encourage songwriters to keep a notebook of song ideas and to start collecting fragments of lyrics and music. Someday your music career might hinge on being able to write a solid album of songs on short notice, and that job will be a lot easier if you've provided yourself some material to start with.