"Trust an expert -- trust someone with experience." And since I started to write about the age of 11 and think I never managed to finish a story until I was seventeen, I think I have some experience here.
So -- some advice. (Free, and worth every penny.)
The first piece is, if you have discovered like me that you get half way through your stories and don't complete them, take a whole stack of those stories and sit down to re-read them. Do not do this from memory. Re-read them. And something beneficial may come leaping out of the stack at you. (At worst, you will have lost a few hours.)
The second is that if your stories just peter out and don't go anywhere, try outlining them. No, not the sort of outline your English teacher would approve of. Write down the events of the story in order. You don't have to keep them all on the same level. (Mine tend not to be legible for anyone but me. 0:) Terry Pratchett once described writing as
(I will, however, note that this advice is not very useful to people who find they can't stick to an outline. Such people exist; they have told me so.)
The third is for people who, like me, frequently can't stick to a piece. You do not have to go on to something new when something goes flat on you. Go back. Circle 'round and pick up the last piece you dropped because it went flat. Or the piece before that. They can come back to life, you know.
And do not decide that a piece has gone flat or hasn't come back to life until you've actually taken a stab at getting words on paper (or screen). Sometimes the flat feeling is just vacuuming the cat.