Here's some tips i came up with when I need motivation:
1) Start with an outline. Knowing the story beforehand and referring to it can give you more confidence in where your story takes place without being disjointed. I can't write without an outline since if I write without one I can't write for fear I don't know where my story's heading.
2) Just write. Hemingway once said, "The first draft of any piece is worth s**t." In other words, everyone makes a crappy draft. Even Shakespeare had to revise. You can edit later.
3) Don't strive for perfectionism. Sometimes this is main reason we don't write. Remember: everything man made is imperfect. Still, with a few errors something can be a great piece of work. The trick is to banish what I call your inner critics, your inner self that be saying, "You're not good enough," or, "This piece is clearly second-rate to the other writers." Banish them; don't pay heed to your perfectionism. Even Shakespeare, in some parts, had weak areas in his writing. The way to reassure yourself is by knowing you can revise it, knowing you can improve it.
4) Write in your voice, or rather, write as if you're the only person who's going to read it. Remember diaries or journals? How come they instantly excite someone when they read it long after you're dead? It's because we hear you in it. In other words, just be yourself and stop copying other writers. There can only be one Dickens and one Chekov and one J.k. Rowling. Don't make the mistake Paolini did writing like Tolkien when in fact there can only be one Tolkien in this world. Just read any book that has the same world as Tolkien and you'll find out only Tolkien's world sound original and exciting. Usually by copying someone else's voice do we lose motivation to write. Can't be everything to everyone. Trust me, it'll be easier if you do it this way.
5) About outlines: if you don't write best by using outlines, then write without one. No need to hinder yourself with something that won't work for you.
Here's an experiment if you don't know whether if you write best by outlines or by the seat of the pants (or spontaneously if that's the better word): Think of two short stories you desperately want to write, and I mean desperately. For one, write an outline for it, and for the other just start writing it form the beginning. After you complete the two stories, think which method was easier for you, which method caused you the least problems and helped you produced better writing. That's your method.
That's all the advice I have on motivating yourself to write.