So I'm new to this thread, but I felt I should give my biggest tip for writing a story: planning. Obviously, I'm not saying you have to have every single detail of the story visualised in your mind before you begin. But if you can get a rough estimate of how the story will progress, and work out roughly how much writing you'll need to do to bring the whole thing together, you'll find fleshing the plot out a lot easier.
For example, when I wrote my first story for FanFiction, I made the whole thing up as I went along. Over a year and a half later, I finally finished it, at a little more than 30,000 words total. For my latest story, I decided to take a different approach, planning out the plot carefully before hand as opposed to ad-libbing the whole thing. This story took me just three months to write, and has over 50,000 words. Both stories have thirty chapters (as do all of my stories), which means that by sitting down and planning first, I managed an average of over 650 words more per chapter, in less than a sixth of the time!
THIS MESSAGE BROUGHT TO YOU BY:
HOME OF THE INTRASOLAR TRANSIT AUTHORITY