Which is an excellent question, as I suspect all of us who write have to deal with this!
For me, I've come to realize that I pretty much always feel like writing... I just sometimes feel even more distracted by other things going on/nervous that what I'm about to write won't turn out well/etc., which overwhelms the wanting-to-write feeling. But once I get myself in front of the laptop and get to it, I usually feel a heck of a lot happier than when I was procrastinating before. So the trick is getting myself to sit in that chair and open up that laptop.
Sometimes all I have to do is remind myself that I will probably feel happier once I do, and give it a shot. (This is even easier to manage when I have a limited amount of time to write in, because I have guilt--the thought that I might not get any writing done at all--motivating me as well.)
Sometimes I can't quite manage to just convince myself right off, so I putter around my apartment. If I'm not going to write yet, at least I want to be productive, so I get myself doing chores or other things that need doing. I do not let myself turn on the desktop computer with the internet. Once I get on the internet I can procrastinate for ages. I also do not let myself pick up a novel. If I do I may not want to put it down until I'm done. But as long as the internet is off and my books stay on the shelves, it doesn't take too long before I get bored enough of puttering and chores that I can get myself into that chair.
I think one of the keys is knowing the things that make it easiest for you to procrastinate, and making sure you have access to as few of them as possible when it's your writing time.
For the occasions when I sit down with the laptop, open up the story... and still don't feel like writing?
I read through the stuff I wrote the day before, and/or the earlier draft of the story, and/or my outline notes, to remind myself what scene I'm supposed to be working on and get the story gears turning in my head.
I listen to the "soundtrack" I've made for the book. Sometimes I'll put on a particularly relevant song, close my eyes, and imagine a TV opening or fan music video for the story. That can get me both in the story mindset, and excited about writing it.
I force myself to write. One sentence at a time. As long as it takes. Frequently after a paragraph or two like this, the words start flowing again.
If I absolutely still feel completely wretched after about an hour of forcing it, and all of the words I'm forcing out still seem like garbage, I let myself stop. Doesn't happen often, but there are the occasional days when the writing part of my brain apparently needs a break.
How about the rest of you? How do you get the creativity flowing when you're just not feeling it?
(P.S., I continue to be open to questions and topic suggestions over here.)