The best advice I've gotten, taken, and given on how to get yourself to start writing is this:
Not very profound or poetic, I know, but it works. So, here's something to help us all overcome out doubts and let our procrastination skills get rusty: a prompt. Open your word processor on your computer or get a pen and paper, which ever you prefer, and put down whatever comes to your mind when you read this. Turn off your inner editor - don't worry about spelling or punctuation or grammar - just write for 5 or 10 minutes. It will help - I promise.
Just start writing.
So how did it go? What did your imagination discover?
A child comes up to you on the street and says "Come quick." You go and find... what?