I tend to have writer's block around 364 days out of the year. On my one good day, I'll get a decent idea for a picture book. It doesn't just pop into my head. It comes from a brainstorming session with whoever is my current writing partner. Right now, that's Becky. We throw out ideas until one resonates with both of us. This can take forever because Becky and I are very different. (Becky voted for McCain-Palin... enough said?)
But even after we have a solid idea, we often have trouble getting the first few lines down. This is where JOPs come in handy. JOP stands for "junk on paper'. Becky and I set a timer for ten minutes and each just free write. We write whatever comes into our head without censoring. This is super hard for me, but I just tell myself that I am not going to show it to her when I am done. When time's up, we compare notes. One of us almost always ends up with something that we can use as a jumping board.
There is something about having a short time limit that forces me to ignore that annoying critical voice in my head and just get words down. I am often amazed at what comes out. I'd share one so you can see a real-life example, but Becky won't let me (See, I told you we were different!)
What about you? How do you get a first draft started?