I use Hitoshi Doi's episode summaries, the Script Crypt, and Alex Glover's manga translations to make sure I depict characters' actions accurately. I also will re-play a video of SM over and over and take notes, if I'm basing the story on that episode. URLs follow:

Hitoshi Doi
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[THE Anime authority! :-) ]

The Script Crypt
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[For the Japanese episode scripts]

Alex Glover's translations
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OR:

Sexylyon's Chiba Mamoru/Tuxedo Kamen Page
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(2) Avoid, if at all possible, adding a lot of new characters, such as the infamous Sailor Earth, unless you can ensure all the existing characters get lots of the action. We already have the 10 Senshi, plus Tuxedo Kamen, and all the supporting characters. If you can't do something with that many characters, there's something wrong! Also, avoid author inclusion fanfics (putting yourself in the action) unless you're writing something humorous. It looks very narcissistic when you do that, and seldom works. And crossover fanfics are also diffficult to do well, unless they're done as a comedy. Again, what's wrong with the SM characters we already have?

(3) Try not to change the characters too much from the original, unless you just like shocking the readers. Don't, for instance, have Mamoru/Darien in a torrid affair with Naru/Molly as one fanfic author recently did [PUH-LEESE!!].

(4) This is my personal opinion, based also on my readers' feedback. Romance readers are happier when you don't make either of the leading characters in the romance act obnoxiously, even if they'll kiss and make up later. For instance, I don't recommend that you have Mamoru slap Usagi around, or insult her, or that Usagi acts too whiny, petulant and immature. And please, no dark fanfics! So many of my readers say they are tired of the downer dark fics. And a little comedy and humour will be welcomed by your readers, who have come here to be entertained.

(5) Use a spell-checker, and grammar-checker, if available, and have someone edit your work, before you submit it for publication. Yes, it DOES matter! A lot of spelling and grammar mistakes are distracting to your story.

Sayonara!

Pandora-sama