To wit, the best advice I can give:
1. Technique, oh ye gods, technique.
You've heard it a million times before. People don't like to read stories that are misspelled, rife with errors, or so horribly formatted as to induce optical illusions in the reader. SO WHY THE HELL DON'T YOU PEOPLE GET IT? SPELL CHECK, SPELL CHECK, SPELL CHECK! AND THEN GET ANOTHER PERSON TO READ IT!
There is no excuse, in this day of word-processing programs and even e-mail programs that will check your words, not to put out a story that's at least lexically correct. More than 5 typos (excluding homonyms) will sink any story under 20 pages.
However, on rare occasions typos can be your friends. Sometimes dippy Serendipity graces the most putrid bile. My favorite example: someone once intended to write "star-filled," but through a typo it came out "star-fill." Though it didn't make sense in the context, I took the new noun and used it a few times. Verbing words can have interesting effects.
But you have to be careful-a story can be spelled correctly, but the words you've selected don't mean what you think they mean. Example: wont, won't and want look the same, but they obviously aren't. Beware homonyms-get someone else to read your story, just to make sure your vocabulary is accurate. Also, read your story aloud to make sure the words don't sound stupid together. Assonance and consonance can work for you or against you.
When selecting words, use variety. I happen to be very fond of archaic words, as long as they're not too archaic. (More about confusing the audience later.) Why use "red" when "crimson" or "incarnadine" carry so many more connotations? The only limits are your vocabulary and your knowledge of connotations, and these can be rectified by a good thesaurus and a dictionary.
Now grammar, that's something different. Here we get into issues of style and dialect and dialogue and deliberate mistakes for effect. (See, you probably hated that last sentence. But technically it's all right, and it used to be my favorite style. Also, it rhymed.) Your teachers should have learned you grammar long ago, and I am not going to teach it to you, because I can't. You wanna know why? Because I didn't learn grammar right. I can't talk English as good as I could if I'd been learned it when I was young.
Heh, heh.
The best resource for this area is, naturally, the dictionary. After that, "The Elements of Style" is good.
2. Know your source material.
Sigh…how many stories have I read containing glaring inconsistencies? Lots. And they're things that could be avoided with even a little bit of research.
For example: DBZ doesn't take place in Japan. It doesn't take place on any Earth we know--the dinosaurs should have tipped you off. Saying that there are Americans and Japanese is just silly.
There are plenty of other examples out there, but thankfully there are lots of information sites dedicated to anime shows. Spend an afternoon perusing them, and you'll cut down on lots of mistakes.
3. Remember Occam's razor.
Occam's razor says that all other things being equal, the simplest explanation for a problem is usually the best. Given the choice between a story written with just the series' characters and locales and races and a story that invents all-new creatures and stuff to tell the same story, I'll take the former.
Now, if you have to have the new people and situations, otherwise you have no story, then that's fine. As long as it's handled well, that is ^-~.
4. Be original.
If I have to read one more story about an ultra-powerful new girl (Saiya-jin or otherwise), I think I'm going to be sick. Likewise for those about a terrible villain with multiple swords. It's been done. Stop doing it.
5. Don't rush yourself.
I'll admit it: I procrastinate. There are parts of stories that I've had the ideas for for months, but I still haven't written them. Why? Because I don't really see them. Unless I get a bit of dialogue or something in my head, or unless something demands to be let out, stories don't get written.
This is a good thing.
Stories that don't want to be written right now have a way of making themselves inaccessible. It's not just writer's block; it's that the stories aren't there. They'll only come out if they're forced. And there's nothing sadder than a forced story.
Remember, you get writer's block for a reason: you are either tired, or you have nothing to say at the moment. Unless you're a pro working under a deadline, nothing bad will happen if you don't post anything today.
6. Refine yourself!
Write a story, right now. Go ahead, I'll wait.
OK. Read what you've got. Would you post this on your forum of choice, exactly as it is? Not bloody likely.
What was the point of that? Only that you should edit yourself a lot.
I'm not proud of myself, but I've been working on a single story since October. According to MS Word, I've put this poor story through several hundred revisions. I think I'm done, but I'm not going to put it up until I'm damn sure. This is an extreme case, but I think I've made the right choice in waiting.
The point is if you're going to put something up, it might as well be the best you can possibly get it. Give it more than a cursory once-over--really pay attention to it. you're doing this for fun, but also for love of a series. And you owe a series you love your best effort. (Not the audience. Technically, you owe them diddly. Some polite concessions would be nice, though.)
7. Have plots make sense.
Chekov once said, "Don't put a gun onstage in Act 1 unless you plan on firing it by Act 3." Though I've paraphrased the original (he was Russian, after all), you get the meaning. If you are going to put something in your story, you should do something with it. If it is meaningless, then snip it out.
Fiction is not real life. Real life has loose ends, and makes no sense, and has no meaning. Good fiction leaves no loose ends, and things happen for reasons, and (according to Jung and Freud) it has some underlying psychological meaning.
There's nothing more maddening than a loose end when it comes to evaluating a piece of work on its own. (Of course, loose ends are what fanfic authors base their lives on. That's not what we're talking about.)
Plots have three basic stages: exposition, climax and denouement. They should be logical--the actions of the beginning should result in the consequences of the end.
And no Deus ex Machina!
8. Write for you.
Never, ever, ever ask someone to give you ideas for your next fic. If you have no ideas right now, don't write. There are plenty of other ways to occupy your time. TV, for example. Or meaningles sexual relations.
Also, only write about things that interest you. Sure, you could write another crappy Mirai Trunks fic, but if you don't give crap one about Mirai Trunks, well…you know how it'll turn out.
Just because people seem to like a certain type of story doesn't mean you're obligated to give it to them. They have plenty of other places to go for a Vegeta and Bulma romance.
It's just good advice. Don't take yourself too seriously--admit the ridiculousness of your creations every now and again. (Coppertone Ishuu comes to mind.) If you don't get into the Good Fanfic webring, so what? Lots of other people didn't, either. You can make your own ring. "People who hate the other ring, but would naturally be very smug about it if they got in," or something like that.
Parodies are OK. Good, even. They cleanse the palate.
Good luck, have fun, and don't listen to a word I say. I'm not right.