I've read a lot of fanfiction. Not all in one fandom, mind you. However, finding good fanfic is very, very hard in any fandom. On the other hand, bad fanfic outnumbers blades of grass in some states.
class="norm"With that in mind, I sat down and typed up my best advice for writing fanfiction.
1. Writing for You
I'm all for fantasizing about running in to save your favorite characters and sleeping with the good-looking ones. I even have a few like that written down. But before you comment, go look at my fiction section. I'll wait.
You're back? Good.
Notice how the only Mary Sues in there were the parodies. That's because as much as I love being the hero, it doesn't make a good story. And the only stories of mine that I put up are the good ones. Well, the ones I think are good.
But wait, you say! What if what I think is good is actually badfic? That's the tricky part, isn't it? Sometimes what you honestly need is a second opinion. After all, if you've invested time and effort into your story, you don't want it to turn out badly. You want to have produced a great story that people will love.
A second opinion from someone you trust to tell you the truth will usually help sort matters out. The trick is to find someone who will tell you the truth and won't just squee because you wrote it. After that, though, it's up to your judgement as to whether or not you post it.
However, why bother posting a story for other people to read, if you're not going to give them a good story? I'm sure you can argue that you don't owe the audience anything. And you're absolutely right. But the fact of the matter is, if you're writing stories for the warm fuzzies you get with feedback then writing a bad story is not the best way to get good feedback.
2. Conflict
All good stories have conflict.
The hero's wife is stolen, and the hero must kill a demon king to get her back. The evil empire sends their troops to a settlement and they kill the hero's aunt and uncle, while searching for something that the hero found only a few days ago. The heroine must abandon her way of life to protect her child and be with her lover.
All those stories have some form of conflict. That's what makes them interesting.
There are three different kinds of external conflict: character vs. character, character vs. society, and character vs. nature. Internal conflict is a struggle within the mind of a character, over opposing needs and desires. Use at least one type of conflict in your story.
Don't try to make things too easy for your hero. If the hero can just walk all over the bad guys, then why doesn't he? If the problem is, at best, trivial, why are you telling the story?
There are certain genres of fanfiction where the problems are trivial - days in the life and fluff, come to mind. That's okay, though, because that's what those stories are designed for. Sometimes the insight into the character is interesting enough to carry a short story. However, novella-length or longer stories need conflict. Series need conflict.
3. Know Your Characters
Most people don't want to read a story where the characters are wildly out-of-character. They want the characters to act like they would based on their personalities and histories.
So no matter how much you want him to, Saonji is not going to forget all about Anthy and obsess over your new character. That's just not the way he would act, given his history. It goes against all his reasons for obsessing over Anthy.
Characters always have a reason for what they do. It could be stupid, but there is a reason. Utena didn't just wake up one morning and decide to be a prince. She was rescued and comforted by a prince when she was very young. Her awe over the prince was what made her decide to become a prince. Her honor and nobility was what made her a good one.
If you don't know why a character does what he does, look at the source material again. If that doesn't help, try talking to fanfic authors who you think write the character well. Most fanfic authors don't mind helping others write better. Read stories that you think embody the character. Study the character as other people see him, as the creator sees him, and as you see him.
4. Love, Lust, and Friendship
Romance is a great way to introduce conflict to your story. However, messing with the pairings thoughtfully put together by the series creators' is a bad thing. Those pairings were created for a reason and are an integral part of why they do what they do. Would Bob have tried to split himself from Glitch if he hadn't loved Dot so very, very much?
If you don't think you can pull it off, then leave the canon couples alone. They have enough trouble as it is.
Do remember that love is different from lust, and the characters don't always know the difference. Also, in some cases, the characters are just really good friends. They may be closer than lovers, but they don't have to be lovers. This is another case where you have to study the characters.
5. Plots Must Make Sense
Don't put something in a story just because you think it's cute. If it doesn't further the plot, snip it out. This may seem like a harsh rule, but you'll find that as you become a better writer, you'll understand it more. And when you understand a rule, you can learn when to break it.
Don't have the hero's car break down when it doesn't affect the characters in any way during the rest of the story. Don't introduce new characters that are just there to look cute. Especially don't wander off into chapter-long digressions about whaling technique.
On the other hand, don't pull things out of your ASCII. If Matrix can go Super-Saiyan in the Game, at least mention that he rebooted into Saiyan armor.
6. Grammar, Spelling, and Punctuation
Considering that most of your standard text editors have spell checks these days, why are so many stories rife with spelling errors?
If you know you have problems with spelling, don't use Notepad! Compromise your integrity and use MS Word.
If you don't know what a word means, look it up in a dictionary!
Use a thesaurus! Read the "Elements of Style"! Buy a college English textbook!
There are so many ways you can make your story more readable if you only make a little effort to use them. Make that effort! It's worth it!
7. Beta-Readers
Beta-readers are the people who take the time to go over your story line-by-line and try to find all the problems. Then they tell you about them. Beta-readers are not the people who read your story and tell you how great it is.
Note the difference.
8. Get Over Yourself
Be polite to your fans and answer their questions. At least think about your negative feedback. Sometimes it's actually important. Don't scream and throw a temper tantrum in your author's notes. It's unbecoming.
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Remember, people, take what you need and need what you take. I'm not always right, and I'm terrible at explaining things.