Passion to write

There are a lot of days when I have a real passionate desire to write, and I just HAVE to put the words down on the paper.  What I need to learn is to harness that passion because, unfortunately, there are a good many days when it's difficult to get even a trickle of enthusiasm rolling.  

I have to wonder how to cultivate passion in my writing and in my desire to write.  What do I do to get it going.  I guess I just need to write.  Put one word after another on the paper and hope that at the end it's what I want to say.