I've found that when I get stuck I can get myself out of it if I just sit down and list what I want to write about. I don't worry about details or even if what I'm jotting down makes sense. Just write.
I participated in the National Novel Writing Month last November and managed to write over 50k in 30 days. (Prior to that the most I had written was 14k in 3 months.) Having a deadline looming over really helped force me to sit down and put in the effort. So make goals and deadlines for yourself and stick to them! Start off with small goals that you can easily acheieve, (like 300 words in 15 minutes) and work up to bigger goals (like 2000 words in a day).
Today I actually managed to write 2044 words within 2 hours using that method.
Written Kitten is also a fun website to use for motivation, you set a goal number and when you reach it the website gives you a picture of a kitten as a reward.
Write or die works on a similar concept except it punishes you by making annoying noises when you stop typing.
My last trick is actually the program I use to write, it's called Scrivener and is an amazing tool for writing. It has a lot of features, but I get by with just the basics and wonder how I've ever managed to write without it. It's a little expensive, but you can get a month's trial free to play with. I used the trial during the writing contest and having dedicated software for my writing helped get me excited to write as well.
Anyway -- there's my wall of text for the evening.