Get rid of nine out of ten of your adverbs. Most of them are unnecessary, and the rest can be substituted by a more specific verb or adjective. I like to use ctrl+f for “ly.”
It’s normal to use contractions in fiction, especially in dialog. A story that uses no contractions at all reads like a term paper written by a college freshman.
In 95% of all situations, you want your dialog tags to be unnoticed by the reader. Simple words like “said” and “asked” are your friends. Most of the time, however, you don’t need a dialog tag at all, as it will already be clear who is saying what.
Use the names of your characters! The rule of thumb in English is to avoid repetition, but the names of people are an exception. This is especially relevant in situations when pronouns can become a problem. For example, instead of “the blond kissed the dark-tressed man,” just say “Steve kissed Bucky.”
If you’re using a particularly flashy word, take care to only use it once. If someone’s eyes are described as “crystalline” once, it’s striking. If someone’s eyes are described as “crystalline” more than once, it’s silly.
It’s not the nineteenth century anymore, and page-long paragraphs have fallen out of fashion. When in doubt, start a new paragraph.
Forget what you learned about structure and formatting in high school. Look at professionally published online writing and take note of how it handles things like indentations and spacing. Fanfic is a reader’s market, and you’re going to lose a lot of potential readers if people click on your story and are confronted with a strange and confusing layout.
Do your research on specialist terms and modes of address, especially if you’re writing historical fiction or historical fantasy. Be especially careful when writing about a culture you’re not already familiar with, and try to consult more than just one or two sources.
On that note, do you really need to wax poetic about the color of someone’s skin. Do you. Really.
Don’t be afraid of being “formulaic,” but don’t feel as if you need to follow a given formula laid out in a writer’s guide, especially if it’s a screenwriter’s guide with a male author. You’re always going to be balancing tradition and originality, as well as the expectations of a potential reader with your own self-indulgence. You have to find a balance that works for you, and it’s going to be different in every story you write.
Sex sells. Include a highly specific kink or set of kinks in your story and tag it appropriately, and you will find readers on AO3, I promise.
If you’re obsessed with a rare pairing, scrub off the serial numbers and replace the names so that you’re writing about a popular pairing. Your readers aren’t stupid, but people love what they love, and you might be surprised by how accepting people are of your new and fresh take on an established pairing.
Write that Hogwarts AU. Write that mermaid AU. Write that “the dark brooding hero/ine is actually a shapeshifting dragon” AU. Fanfic is not and will never be a judgment-free zone, but it’s been my experience that even the most niche AU stories can find an audience. Treat yo self!
Try to finish things. If you’ve only written bits and pieces of your dream novel, post them as their own separate short stories. If you write three chapters of a fic you planned to be thirty chapters long but then get stuck, figure out a way to wrap up the story in just one or two more chapters. It’s always good to end on an emotional climax, like “and then their eyes met” or “and then they left on their journey.”
Leave kudos on other people’s stories, and leave comments if you can. Even a short comment, like “I love this,” will be appreciated. No writer writes alone, and this is a great way to make friends through your writing.