Tips in writing EVA fiction
There are, of course, universal standards of good writing, which I can't get into here. However, I can cover some specifics important to this series.
To begin with, know what you're writing about. Watch as much of the series as you can before you start. Really. To truly write it, you need to get a feel for it. First of all, it's not your usual anime. It's something else. It is intended to make you think and feel; it would be an injustice to the anime to write a serious EVA fic that didn't at least try to do the same. Secondly, as has been mentioned a few times, this thing has a tightly defined continuity. It's not like Ranma, or Sailor Moon, where the storyline is relatively long and rambling, and you could fit things in rather loosely. Not at all. In those two shows, you can refer to certain plot points by a label of their occurrence. In EVA, you refer to them by the episode.
Then, when you start writing, have a clear intent in mind. If it's going to be humorous, make it humorous. If it's serious, make it serious. Don't try to combine the two too much. They don't mix too well. This is a difficult point to define, though. Take Evanjellydonut for example: it is meant to be a spoof. However, I have inserted some serious plot points and ideas to think about to the story, such as the father-son relationship, and the potentially dangerous situation when Bardiel infected Unit 02 and attacked Rei, in 00. Still, the feedback about this, making a "spoof that is not a spoof", has been almost entirely positive, so I must be doing something right. The important thing is, among all this seriousness, I have remembered the jokes, the quasi-non sequitur dialogues, that touch of silliness which carries the humor through. The wrong way to do it is to have completely incongruent sections--a scene that is all laughs, followed by one that holds no humor in it in the least. This is particularly true if the humor is of a parody type, rather than one that could fit in naturally along with a more somber plot. I've seen this done a few times, and it really did make the fic sort of mentally fuzzy. (I mention this specifically for EVA because, for some reason, it lends itself to spoof- type fics as well as serious, cereberal bits. It is probably due to the nature of the anime, being so heady and "up-there", begging to be made fun of--one of the prime reasons I wrote Evanjellydonut.)
Next: be very careful. This is a show full of mysteries and secrets. If you're going to reveal one, make sure you know what you're doing. It's best to follow the canon, for the most part, in matters such as the nature of the EVAs in respect to their relationships with the pilot. Know that in actuality, only Shinji was born nine months after the Second Impact, despite what is said in the series--and know what that entails. (I suppose this does somewhat fit in with the first paragraph.) And be prepared to explain things that you make up yourself, if you decided to depart from the canonical storyline. It is easy to be lazy, again, with this show; it is very intricate and twisty, and there is the temptation to just put stuff down and not bother to elaborate. I myself am guilty of this, I suppose; I use the manga characterization of Shinji for some of my stories, and did not say so....