I don't like to post quotes, because nothing seems so cliché as to confess to being unable to think of something yourself.
But every now and then, I come upon a good one. Too good not to repost, because it is 100% true:
If you're going to be a writer, the first essential
is just to write. Do not wait for an idea. Start
writing something and the ideas will come. You have
to turn the faucet on before the water starts to flow."
~ Louis L'Amour
Something about the very act of writing--physically writing with a pen and paper, not typing--lets ideas creep their way in. You start getting more and more ideas--and that's when you want to turn to the computer to get them all down before they start going away.