1) Stick to one theme, don't go off with something you never mentioned before. That's confusing to the reader.
2) Use spell check, that's what it's there for.
3) Use spaces after sentences please! That way it's easier on the eyes.
4) Don't use crossovers too often because if you write well, but the reader hate X-Men if it's a Sailor Moon/ X-Men crossover, they won't read it.
5) Don't make the actual sailors attacks up a lot. For example, I read a fan fiction that was supposed to have the actual attacks, but they were all messed up and it annoyed me a lot. If you make the characters, make up the attacks all you want. If not, don't. It is confusing!
6) Make dramas a bit funny. I hate not getting at least one person for comic relief in a story. Maybe just two or three funny lines can cheer up the person. Like in one of mine, it starts out funny, but gets kinda sad and dramatic.
7) Stick to one tense, even I don't sometimes, but really try to, it will make you an even better author!