Author has written 7 stories for Beyblade, Naruto, and Harry Potter.
Aspiring Tips for the New Author
This is mostly stuff I derived from my experiences with writing Naruto fanfics, but it can probably apply to most other fandoms.
1. Make your Story Premise Interesting and Partially Unique
Fanfiction becomes fanfiction when it deviates from the canon storyline and background in some way. For example, in my story, Uzumaki Naruto: Shapeshifter, I give Naruto shapeshifter powers, which he didn't have in the manga. This is interesting, because being a ninja, shapeshifting would be a very useful talent to have, and so the premise draws in readers.
At the same time, it has to set itself out from the crowd. Some premises that once drew readers in by the hundreds have since dulled out - for example, in Harry Potter fanfiction, having an evil!Dumbledore who sealed the Potter will and put Harry with abusive relatives, and Rebel!Harry who decides to undo all of Dumbledore's plans. While good writers can still make it work, to first attract readers you need to draw them in, and if they read a summary and see 'ho-hum another manipulative Dumbledore fic' they won't even click on your story.
2. Don't Rewrite Canon, and Changes have Consequences
One of the most annoying things to a reader is reading something that is basically a written adapation of the manga, except with the occasional modification so that Naruto acts 'cooler', or for character bashing, or so forth. Even if you insist on keeping most of the same scenes, try to change the dialogue considerably - for example, if you have a roof meeting with canonical Team 7 introducing themselves, change what they say. Have Kakashi preach teamwork. Have Naruto and Sasuke banter a bit over some random tidbit Naruto mentions.
A serious change earlier on can be to leave out the Wave mission entirely. Maybe Tazuna explains the situation to the missions office and gets a team of jonin with promises of Wave paying for the mission in a series of payments instead of having a C-Rank mission. Maybe Naruto mouths off a day earlier or a day later than in canon and Team 7 gets a mission elsewhere. Don't rely on canon like a crutch.
Another drastic change can be the third exam of the Chunin Exams. Out of the 26 teams that Anko mentions having gotten past the first exam, it doesn't always have to be the exact same seven teams who make it past the Forest of Death. Take out Team 10 who lucked out, and you suddenly only have six teams. Maybe some other random Suna team makes it in. You can also do some major character deaths in the Forest of Death if you really want. The third exam is similar - taking the exact same ten match-ups as shown in the manga and anime and basically doing a few blurbs for most of the matches and writing out Naruto-Kiba, Nej-Hinata and Gaara-Lee extensively gets boring to read. Spice up the match-ups a bit. Maybe Hinata fights Naruto instead. Think how much different things would play out if that occurred.
For the second part of this point of advice, changes have consequences, think about how your story premise changes the storyline. In my story Uzumaki Naruto: Shapeshifter, because he has the power to shapeshift, Naruto spends time toying around with his ability. This distracts him from people who occasionally shun him, and because he is experimenting, he doesn't have time to act out and prank for attention. As a result, he has a calmer personality, and he makes friends with other orphans. This snowballs into the teachers who think he would be a better fit with orphan friend teammates rather than Sasuke and Sakura, who he barely knows.
In another story of mine, Circle of Life, Naruto is deposited into the body of the Kyuubi jinchuuriki in another reality. Here, a few things are different between this reality and his home reality, and they occurred more than a decade before the Kyuubi attack. This means everything - the Konoha Twelve are never born, because two parents have sex at a slightly earlier or different time, resulting in a child with a different genetic mixture. For example, Fugaku and Mikoto have three daughters before they finally hit a son, at which point they finally stop because Fugaku finally has the son he wanted. Granted, this is an extreme example, and most people should stick to canon characters even with For Want of a Nail, but hopefully you see my point - even slight changes must have consequences.
That ties into the last part of this point - if you start changing stuff, stick to it. Too many authors appear afraid to stray off the path of canon. If you overpower Naruto to the point that he beats up Orochimaru in the Forest of Death, Oro isn't going to sit back for the invasion. He's going to find new allies to help havoc for the invasion, or maybe he'll pull some dirty tricks to help achieve victory like poisoning Konoha's water with a toxin that slowly eats away at the chakra coils the night before the invasion, which results in bigger losses for Konoha's forces.
3. Try Not to Character Bash
Simple as that. It might get you more reviews when you bash Sasuke and Sakura, but it's not good writing. Following into point five, don't be stupid, either - just because Kakashi pushed Naruto onto Ebisu while going off to train Sasuke doesn't mean he's a bad sensei; rather, Kakashi is the only person in Konoha who can teach Sasuke how to use the Sharingan, while there were likely dozens of people who were more than capable of teaching Naruto about strategy (not tactics) and chakra control, both of which he desperately needed.
One thing in particular that bothers me is how people tend to demonise Neji over the course of the preliminaries in the Chunin exams, and have Naruto ignore the whole Branch House seal. Naruto in canon has empathy - even if he arguably has a worse hand dealt to him in life, being the Kyuubi jinchuuriki, he shouldn't be 'yeah well I have a worse life than you so suck it up'. He should be '
4. Avoid Fanon
Anko is a psycho who secretly deep down is hurting for positive attention. Kurenai hates perverts. Kiba is a horndog rapist. Hinata isn't good at Jyuken because she doesn't have the right body build or the right elemental affinity, just make a few changes and she's a genius! Lee has a chakra defect that prevents him from using ninjutsu or genjutsu (in reality, he simple has no talent for either arts). Minato was a goofball while Kushina was the straight-laced one.
All of these are constructs of fanon, and more beside. Some of these work alright if done properly, but if you are abusing them, try to avoid them.
5. Be Logical
Instead of explaining this, perhaps it would be easier to start with an example. Lots of people tend to make Kankuro a mook in the exams. For example, if he's fighting Neji or Hinata, the two will see he's using a puppet and magically spring in close and disable the real Kankuro.
Kankuro is a puppeteer, and as shown by Sasori and Chiyo, puppeteers tend to carry poisons around. Kankuro's puppet should spit out an odorless, scentless, colorless gaseous sleeping poison and knock out Neji or Hinata in fifteen seconds.
Just because something might be easy to write doesn't mean you should. Don't be lazy - think about what you're writing. If a character doesn't act logical or if a scene doesn't play out as circumstances dictate, look it over and come up with something to fix it.
Less strictly, try to keep up to date with whatever the 'facts' are of your fandom. In Naruto, Naruto is 12-13 pre-timeskip, not 14-15. The 'you've failed twice already!' line in the first chapter is exactly that, a line in the first chapter. Similarly, the Leaf doesn't turn out just 9 new genin a year (as suggested by Kakashi Gaiden, where it is mentioned that in one battle Iwagakure had more than one thousand shinobi - for the Leaf to match that number at 9 genin per year, they'd have to churn out 9 genin for 111 years, with nobody dying).
6. Don't Shoehorn in Romance Pairings
It seems like every story needs to have some sort of romance. No, they don't. Especially if it's a comedy, you don't need to have a main pairing romance going on; you can feel free to have a friendship with implications of developing romantic feelings, but don't do an actual romance. It detracts from a lot of stories.
Also, remember point five if you DO do a pairing. If your Naruto decides to run away at age six or age eight (early childhood) and ends up in another hidden village (or if he ends up the Kage of another village), he should NOT be paired with somebody from the Hidden Leaf.