I have a hard time not finishing, too.
I have a friend that suggested you start with the ending, almost J.K. Rowling style, then edit the ending a little if the story doesn't quite end up at that destination.
she also suggested that you draw some kind of map to where you want it to end. I know I have a huge problem with my plot going all over the place, and I'm obsessed with back stories, so this helps me stay on track.

I'm deathly afraid my fan characters are Mary-sues, so I always come up with a weakness that would contract her strengths. Nothing like 'she's afraid of spiders' but a genuine, debilitating issue.
The main problem with a lot of oc's is that 'this person is so tough, but I'm so cute and I'm going to let so and so fight my battle for me'.
For starters, it's almost impossible to be tough and cute at the same time, especially for girls. It's the most unrealistic thing I've encountered with sues.
I've seen some awesome sues who were pretty, cute, and smart, but they were literally cold hearted Nazi's.
The hardest part is finding a balance; someone to whom the reader can relate to as well as not making them too powerful/weak, too pretty/ugly, or too evil/good.
The ones that drive me nuts are the ones that are too good.

Anywho, there's my two cents, hope it helps a little![COLOR="Silver"]

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