Author has written 4 stories for Harry Potter.
I am LordGranger. I am mostly reading HP fans, usually ones involving my favorite character (Hermione). But I am also reading other fics and I am planning to write some. (like Naruto, Death Note etc.) I am mostly writing and reading Harry/Hermione stories at the moment so be warned now.
I like writing but my English is not really good so I do make a lot of mistakes. I hope to get better with time.
Here are some points what I think helps to write a good fanfiction. Don't forget that these are my personal ideas but it might help some of you who wants to write his own story.
1- Make sure you know your characters:
This is a mistake that most of the authors do. They just start a story without analyzing their characters properly and end up changing them with every chapter. Know your characters, especially your main characters with all their strenghts and weaknesses.
2- Find a good title and write a good summary for your story:
No matter how good your story is, there are million others and people won't know how good your story is until they start reading it. To make them at least give a try, make sure you have an interesting title and a good summary. But make sure that the summary explains what you are going to write or you will get a lot of bashing afterwards.
3- Explain your preferences at the beginning:
If you are going to bash a character or make a character more important than he/she is etc. Tell such things at the beginning of your story so if anyone has a problem with it, he can just leave and you will have readers that have similar views as you.
4- Always leave some suspense:
Lots of people might say that they don't like cliffhangers but the truth is, cliffhangers make readers wait for your next update. Let them have an idea what is going to happen next, but not too much. Most of us are writing fanfictions with happy endings and everyone knows that, which already kills the biggest suspense point you have. Don't bore your readers with too much happiness without any dangers. Which brings me to my next point:
5- Keep the balance between good and evil:
I know it is fun to write a story where your main character gets super powered and takes on the world but in a long shot, it really gets boring after a while. If you are writing 'adventure', than keep the balance between main protoganists, the main antoganists and also the side characters. If you make your characters too powerful, you will find it hard to write a way to put them into danger, which will result with making your characters dumber so they make mistakes.
6- Don't change defining characteristics:
Unless you specifically want to bash a character, don't take away his/her most important characteristics. For example, some Harry/Hermione writers want to give everything to their main character, and make Harry suddenly smarter than Hermione. Since it is a fanfiction, there must be a limit how much you change a character or the character won't be believable anymore. A sentence like: "Albus Dumbledore was not that powerfull..." or "Snape was actually weak willed..." or "Hermione was not that smart..." etc. won't be believable, unless these are key plot points and you explain those throughly. You might not like a character and you might want to bash him/her, but do it with style. Don't dumb them down for no reason, keep them strong and let your main character improve so he can take him down.
7- Don't play favorites/ Don't try to be too objective:
If your favorite character is X, feel free to make him/her a main character. This is your fanfiction, don't force yourself too hard to keep the balance of the original series. With that being said, don't over do it. Like I said before, don't just take the characteristics of other characters to give it to your favorite character. If you do that you would have limited readers and even the ones that share the same ideas as you might get bored after a while.
8- Decide when and how you want to end your story:
Writing gets much easier if you know how you want to end your story. Leaving a few small hints for the ending here and there is also a good idea and would make a better story. If you can make your readers say: "Oh, that was mentioned in chapter 2, it all makes sense now" then you are really good at what you are doing.
9- Don't rush!
This is a mistake that I myself did with my current story. If you are writing something against canon knowledge, like a pairing that wasn't in the original story, then make it believable and go slow. Let the feelings develop with time, explain it in detail to not make your readers feel weird about it.