Some days are harder than others. The words don't flow, the page looks at you in a funny way, your story's two-children-short of a gingerbread house and the last thing you want to be doing is sitting down to berate yourself over the next sentence. So, some simple tricks:

WrittenKitten - you write in the text box. Every hundred words (or whatever interval you set) the image box beside it presents you with a new kitten. The work isn't saved, so make sure you copy-paste anything you want to keep into a file on your computer, and your distraction-to-motivation ratio may vary (I personally found myself adding random words just to get the next kitten, instead of writing) but hey - it's adorable.

WriteOrDie - a tool that prompts you to keep writing with anything from gentle reminders to deleting the words you just wrote as incentive. Since I last mentioned them, they've developed desktop and iPad versions of their original web-app. $10 for the desktop app which, when you consider it's a couple of coffees, could be worth it if that kind of thing works for you.

A more low-tech version is to consider paying yourself to write. But - and here's the important part - you don't get the money until the work is finished - when it's being sent out. Not when it's published - that's not in your control unless you self-pub. But if you're using traditional markets, then you can consider something 'finished' when you've sent it out.

Work out a method that makes sense to you - pay yourself by the hour, if you tend to do a lot of 'behind the scenes' work that doesn't involve actual words going on a page (but 'waffling' does not count as work. And deep down, you know the difference.) Or you could pay yourself by words, or by page, or by scene or chapter. You'll need to estimate how much you'll end up paying yourself in total, and how much you can afford. And remember to actually put the money aside as you earn it - otherwise it's too tempting to reduce your reward later when you feel you can't afford all that in one hit.

Record how much you've "earned" each session and put it aside somewhere. A sock or drawer if you're using actual money, or earmark a certain amount in your bank account. Watch it slowly grow as you keep working on the story. When you've sent out the story, you can have the money - spend it on something that will make you feel rewarded, not something that you'd spend money on anyway. You earned that money, your reward is the ability to do whatever you want with it, guilt-free.

You can use other things than money, too - time, for instance. Maybe over the course of a novel, you earn a nice weekend vacation somewhere. Or time doing an activity that you enjoy, but normally gets left out because you have "more important things" to do. Whatever works for you as a reward; break it down into small increments somehow, and slowly 'earn' them by working.