Do you take a moment to get yourself excited, to try to get yourself in the mood?
I'm finding that if I focus on getting in the mood for writing, and if I feel like I really have to get into it before I can start, I'm less likely to start at all.
If, however, I just start writing, I find it almost magically starts to flow. If I don't think about how good or bad it is, and just trust that whatever I put on the screen is OK and the wrinkles can be smoothed out during editing, and I just let the characters do what they want, the writing almost takes care of itself.
Do I have to think about what I'm writing? Sure. But more and more lately, I'm finding that it's a matter of getting out of my own way, and just trusting in my abilities and characters to be good enough and more.
What about you? When you're frozen by perfectionism, what do you do to get yourself started?