Just because it’s fan fiction (fanfic) doesn’t mean it shouldn’t still be good writing. If you’re going to write fanfic, do it right.

1. Don’t pants it. Too many people — including me, when I used to write fanfic — would just sit down and start dashing something out without thinking. Then, when it gets a few positive reviews, they start expanding it with no set idea for what’s going to happen, how it’s going to happen, or how to keep it internally consistent.
2. Don’t be ashamed of it. You don’t have to be ashamed of your fan fiction, and the more you treat it like “real” writing, the better you’ll feel about it. Do research, write outlines, workshop the story with other fanfic writers.
3. Don’t self-insert. While that sounds kind of gross, what it really means is to keep yourself out of your story. You should absolutely not be the main character (especially if you turn yourself into a Mary Sue), and you shouldn’t be one of the “top billed” characters either. Nor should you add yourself as a character who is the agent of change — ie: the super-hot new Slytherin at Hogwarts who dates Harry Potter and helps him to save the day in all the books.

This is just a brief list. I’m sure you have ideas of your own. And, as I’m sure you’ve guessed, these three items can also be applied to non-fanfic writing.