DO:
- Have a BETA READER; it's always smart to have a second set of eyes read over your work before its posted. Betas can save lives.
- Aim to please yourself first; I know that, for me, I can never finish anything if the entire time I'm focusing on what I think everyone else will like. It's your story, write what you want to write. People who like it will read it.
- Have a title worth remembering. I tend to finish stories more if I can remember the title, and look forward to it. If you name your fanfiction something too vague or too intricate, people will have a hard time pinning a story to your title. Kind of like sticking a name to a face, you know? Make the title short, simple, relate to the plot of the story, and memorable. Example of some unforgettable fanfiction names: Baek in a Box, Anterograde Tomorrow, For the Health and Safety of Mabel Pines, Overgrowth
DON'T
- Don't start a bunch of stories without finishing them. It's okay to be writing one or two or even three fanfictions at a time, but I once had a friend who had twenty-seven fanfictions posted, and only had the first chapter posted of every single one of them. Don't bite off more than you can chew; take your time, the plot isn't going anywhere.
- DO NOT EVER give away your whole plot in your title. I remember when I first joined the online writing community, there were an insane amount of fanfictions called things like, "My boyfriend's brother is my Alpha Mate but he rejected me and I'm pregnant, HELP."
I'm not kidding. I could send you screenshots.
- Don't make the blurb boring, or likewise, give away the story. A blurb is meant to catch the attention of the reader and make them curious about the story; give limited information, but enough that people will want to indulge.
- DON'T write a PLOTLESS story. There are so many stories I see that end up being 70 chapters long but there is NO PLOT. It's just a serious of random- events happening, with nothing actually leading to anything. Avoid this. At all costs.
- DON'T focus on word count. It's important to have a chapter longer than like a paragraph, but definitely don't waste time trying to break the 4k word mark or something. A chapter should have a point, and once the point of that chapter is reached, it should end. Take Stephen King. The first chapter of Misery, one of THE BEST novels, is 12 words long.

Hope that helps! ^_^

Do write whatever you want to write, really. Forget about how you shouldn't add clichés and whatnot. If you want to write a cliché, write it. Write whatever you feel like. If you don't feel passionate about your writing it will shine through, no matter how original it is. So if you're really feeling a cliché story, write it.
And write. Write and read and do some more writing. Practice makes perfect.

Do
-Make sure you characterize your characters! Do not make them one note, which is something I see so often and it's so so so boring to read about. It's too predictable otherwise, and you want your characters to have dimension. Give them a goal, a motive, desires, and three-dimensional feelings! For example, a character isn't going to just be sad that someone died, but he/she could also be angry, in shock, feeling guilty, longing, nostalgic, or even in pain. It's manipulating all these to strengthen characters and improve the flow of the story that makes a fanfiction good in my opinion.
-A significant and well-written plot can make the most predictable stories interesting. There needs to be some kind of struggle for the main character(s) that isn't just centered around all romance or all action. There needs to be a blend for readers.
-Make sure you polish your POV. POV is so so important and often skipped over, but if you skip around too much or don't have a well-established POV, it's almost impossible to follow along and understand the thoughts and feelings of your characters.
-Brush up on grammar skills! English is a tricky language with lots of nuances, so make sure you know the basics like their vs. there and etc. Be sure to look at trickier rules too, because better grammar helps the reader absorb the story better.
-Get a beta! I cannot stress this enough. Betas will fix a lot of the issues that you can't even see in your story. Whether it be grammatical, plot, or something else, a good beta will catch it.
Suggestions
-Get a poster? It's easier to attract people to a story with a nice poster. I suggest a few stores: Petrichor, Timelapse, Anomalistic, Le Noir, etc.
Don't
-Do not use too many cliches or tropes. It makes a story too predictable and boring and if you look at confession sites for AFF, it makes people gag (if you want a good idea of what people hate, go to affconfessions on tumblr or something).
-Don't forget to get outside and experience the world for yourself. Good writing is often built on the experience and research of the writer.
-Do not drop a story! Make sure you're going to see it through the end. Scrap old ones that you aren't writing or can't write anymore.
-Don't forget a hook. Readers needs some sense of excitement early on in your fic in order to keep reading.

People who follow this list well: snowchocolate97, Korekrypta, Emilieee, nyamnyamnyam, sass-hole, and many more. I find them great if I'm stuck on something and I don't know how to write a particular scene. All of their fics are pretty well-written.