Three

Don't write stories in the form of dialogues.

Make sure you know your backgrounds.

If you're doing AUs, make sure they seem legitimate.

If you're doing a mythological book's fanfic, make sure you know the mytholgy behind it. (This is for you, Percy Jackson fanfic writers.)

Don't bullshit your way through the fanfics by randomly adding stuff that contradicts everything you've done previously.