When I write, I don't write it for very long, 10 pages or so at max, but I do motivate myself on few, some would say, weird ways.
First, I write. Seriously, I write, I never type. If I need it on my computer, I just copy it. I don't know, I never get motivated to write when I'm on my keyboard.
Second, music. Depending on what I'm about to write, I choose different music. For fantasy, I listen to folky stuff, sometimes metal. For sci-fi, electronica or music from some sci-fi video games (e.g. StarCraft soundtrack). If I'm writing something more darker, morbid, then dark ambient comes in nicely, or if it is violent, then some brutal metal. (On a side note, I write short stories - later a bundle few of them up...usually. It's a hobby, really. Not that I'm good at it, though).
Third, writing under influence. I'm not meaning drugs (though you can take alcohol). To put it in a better way...
You know those nights when you drink a bit, have a few laughs, girl dumps you, you get your ass kicked and what else not - and you still don't give a fuck? Those nights are great for writing.
When you come home (I prefer walking - to flush bits of alcohol out) you really have a mixture of emotions and of various "aches" (be it stomach, head, butt or whatever), it is a great moment to put them on paper. Sometimes writing without plan in this moments is best, then later, when you sober up, you just clear it up a bit, correct a few mistakes and there you have it - potentially a great piece of work, or a total disaster.
Weird, yes, but you probably know what I mean.
Fourth...well, there isn't really fourth. Take some free, quiet, moments for yourself (e.g. when taking a crap) to think what is going to happen next and so on. You know the drill.
Like I said, I'm a bit weird at it. Not that I'm a loner in the method.