I wish I could say that I have a vision in mind when I start out, but I don't. What usually happens is that I have a bunch of ideas floating around in mind, but nothing that jumps out at me, so I keep pondering. I stare at the piece in the hopes that it will inspire me with an idea for what to do.
At this point, I'm usually anxious to just do something, so I put down whatever design is easiest for me and then go from there.