This is not a new phenomenon, but nonetheless it is a common one, and I think it's worth addressing. There is a general trend on this site for users to post blogs and/or wiki pages for new, ambitious fan-fiction projects. "Check out my new project! It's new! It's a project! I hope to have it released in the next few months!" the posts invariably gush. "It'll be great!"
Well, there's certainly nothing wrong with doing that. It's good to be excited about your work, and SWF exists as an outlet for your (Star Wars-related) imagination and creativity. It's even just fine to never finish fan-fiction projects. There's nothing inherently wrong with that. Either due to lack of interest in the project, shifting priorities, RL, a realization that the work is not what you hoped it would be, whatever--there's nothing wrong with dropping a project.
However, for those of you who want to finish something, let me ask you this:
Why do you want to finish? Are you hoping you'll get more attention if you finish it? Are you hoping that it'll finally tell that story that's bursting inside of your head? Why do you want to finish?
"Darnit, Ataru," you rhetorically say. "Stop being an old codger and tell me how you think I can actually finish my ambitious project!"
If the answer is that you want more attention, forget it. Writing stories for the sake of popularity on this site is generally not a successful path to attention. Write what you want because you want to.
If the answer is that you want to tell that story you've been crafting and molding and dreaming about and fascinated by for years--now here, here we have something we can work with.
The first thing you should do to be a successful finisher of works is to read. Not your own work. Read other fiction, preferably in similar or related genres. Read writers who are better than you. If you like reading other fan-fiction, read fan-fiction, but try and limit your selection to authors who are unabashedly better writers than you. Read canon novels, or Legends, but read. Don't just take my word for it. Reading does not guarantee you will be able to finish, but it will expose you to words, ideas, and ways of storytelling that will broaden your own experience. I learned the word copacetic from Clive Cussler. Have I used it before? No. Can I use that going forward? It's in my lexicon now. JM76's Phantom Rising uses the unreliable narrator well. That's something I can use now for having seen it there (and other places). Having a more diverse store of words, story arcs, and narrative devices increases the likelihood of finding the right one to wrap up that story arc.
The second thing is to have an outline or a plan for the overall arc of the work. Who are the characters? Who changes throughout the arc of the story? Who doesn't? What happens to them? I personally don't have every scene, every exchange, or every resolution set in stone before I start writing, but I do have the overall arc written out before I start writing. A good story should have a beginning, a middle, and an end, and so having those in mind before you start helps avoid dangling story arcs that trail off into the ether . . .
The third thing is to write as often as possible. Even if it is only for twenty minutes. Even if you don't finish the chapter. Even if you end up rewriting that segment. Write as often as you can. It will keep your mind from being too distracted from all of the other things it could be contemplating.
Those three thoughts won't finish your works single-handedly, and honestly, they're not for everyone. SWF is a sandbox. There's no compelling reason insisting you must finish anything. That's part of the draw. However, for all of you would-be completionists out there, I hope you found something useful.