When it comes to the kind of fiction most of us are writing, prose is nowhere near as important as plot, a point many new writers have trouble with. Think hard about how consistent your story is, how your characters act, and whether the overarching plot itself is believable and coherent.
As far as prose advice goes, simple is generally best. Don't get yourself into the habit of overcomplicating things. Certain words have low visibility, like 'said', and can be used as often as you like; don't stretch yourself for different ways to say things. The same goes when referring to a character - pronouns are your friend. Generally, the only time you shouldn't use them is if doing so would make the meaning of the sentence unclear.
Finally, I'll hand things off to some people who are far better at giving advice than I, the Science Fiction and Fantasy Writers of America. Also worth your time is Writing Excuses - if you don't have time to listen to all the podcasts, check out the transcripts. Lots of very good advice that I guarantee will improve you as a writer.
Great information to help people get started on writing their fanfics. I am currently writing a bleach fanfic and the information that is on this thread is a great asset for me. After reading this thread I will have to go back and redo some words from my arc.
Here's my word of advise.
If you're going to use a bleach character.. you need to understand the characteristics behind the character. You must become the character, even if doesn't fit into your personality. For an example. I am writing an arc that's exclusively involves both Mayuri and Nemu including a new character that I've created. For the readers to enjoy your story you must become the characters that you choose. For writers like I... I have the responsibility to get into the mind set of both Nemu and Mayuri and express them by writing about them and to invent a story that my readers can enjoy. I cannot let myself get emotional when Mayuri starts becoming abusive to Nemu by physically hitting her, including verbal abuse. You must do a lot of research on their personalities and characteristics. Do not change anything because the readers will be fickle. Do not allow the characters to be ooc - out of character... unless if it's based on humor.
I've read many fanfics on http://www.fanfiction.net/... mostly with Mayuri and Nemu (Yes I'm addicted to those characters) and at the beginning of the story was great but soon after the writers start changing the personality of the character.
One of the solutions I've found to help me out of the dreaded writer's block and the plot swamp is to re-read what was written. Sometimes writer's block is caused by loosing your way in the story and a refocus by reading your material can clear it right up. One thing I've been doing with my current writing project, is that I write in down key points that happen in that chapter. I've been going through a partial rough draft (roughly 40 k worth of words, not that word count matters) and breaking the mass into manageable chapters.
But as stated heavily in this thread. Plot, Content, & Keep your Characters True to themselves. All three are like the sides of a fire triangle, without one the other two fail to have a compelling story.