1) Make sure the words you're using are correct.
I know that this seems almost as obvious as the standard 'spelling and grammar' advice, but the amount of stories I've read where the author has put 'defiantly' instead of 'definitely', for example, are staggering and very off-putting. I know that sometimes it's difficult to spot when you've accidentally typed a word wrong, since spellcheck usually doesn't pick up on an incorrectly-used word unless it's spelled wrong, so it's important to proof-read. (I used to be incredibly guilty of this, and sometimes when I post things when I'm tired I still make errors.) I’m not saying that you can’t make mistakes, just try to catch them when possible.
2) Don't go into detail about Reader's appearance.
I have come across quite a few stories where it's basically an OC/Canon story, but the author thinks if they slap "Reader Insert" in the tags, nobody will notice. Thing is, if you give a Reader a skin colour, hair colour, eye colour, etc,