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, then chances are, it probably won't match up with whoever is reading the story, thus meaning they can no longer put themselves in the Reader's shoes. On that note, giving them names is also something that pulls you out of the narrative; unless maybe it's a nickname Reader acquires for something they did in-story. Also, to be honest, unless the plot calls for it (say a person with a particular physical characteristic, like pointy ears, is being targeted or is noted as special in some way) the Reader's appearance shouldn't really be terribly important. The whole appeal of them is to be able to imagine yourself in the story, so spend time on that and not on arbitrary physical characteristics.
3) You don't need Abbreviations
There is a huge variety of these things, we've all seen them. There's "____", "(Name)", "(Y/N)", etc. And the thing is, the majority of them are completely unnecessary and it’s often really confusing. I can't tell you how many stories I've bailed out on reading because of the abundance of them. This is especially bad if the story has like a key code in the beginning so you can keep track of what the hell they all mean - like (H/C), (E/C), etc. Putting "you flipped your (h/c), (h/L) hair behind your shoulder" is totally unnecessary when all you need to write is "you flipped your hair over your shoulder" - the colour of your hair (or eyes, or skin, or handbag) should not matter, and the audience will fill in the blanks by themselves anyway, as it is a Reader Insert. Putting in all these abbreviations distracts the reader and pulls them out of the story. With "(name)" it's slightly harder to avoid, but there are ways around it, like, "You heard him call your name" instead of adding in an obvious blank space. Ultimately, keeping it simple is the best way to go.
4) Have a plot
This probably seems obvious to most of you, but I'll elaborate a bit more. I know that there are a lot of stories out there that are popular concepts, like "Vampire!Character" or "Tsundere!Reader", and that's absolutely fine.
I've only recently gotten back into Reader Inserts, and to be honest, I rarely read them on DA. Why? Because younger readers will copy a work they like, then others copy THEM, and so on, and they are all the same. I've read so many stories that have minimal plot and it's like someone just copy/pasted the whole thing and put it under a different account. If you want to do a super-common story, that's fine, but make it your own. Add a twist to it, put something in there that makes it stand out from other stories with a similar idea, or yours will be literally indistinguishable from the thousands of identical ones already there. A great example is a high school AU - so many things you could do with it, so be creative! Try something different.
5) Canon characters are important - don't sideline them.
Again, this might go without saying, but let me elaborate. The whole point of Reader Inserts is to imagine yourself in whatever universe you want, interacting (whether romantically or platonically, whatever) with whatever characters you want. That’s the beauty of it and it’s understandable with so many fandoms to choose from, the notion of a Reader Insert is growing ever more popular. But, just because you are an Insert in the story doesn’t mean that it should be all about you. In a lot of stories I’ve read, the canon characters literally have nothing better to do than talk about the Reader, discuss who the Reader is interested in, etc. (This happens a lot in OC stories too, or sometimes in fics where the characters have nothing better to do.) This becomes ridiculous for a lot of fandoms where there’s usually quite a lot of plot going on for everyone to stop and talk about a non-canon character. Instead it’s a lot better if you find a way for the Reader to interact with the canon character organically, joining them on their adventures, offering a new angle to a story arc, or having the story take place in an AU. Either way, don’t neglect the personalities of the canon characters, because they are probably the main draw to the story. Make sure they are in-character and have interesting things to say/do.
6) Do not spam author’s notes.
Author’s notes belong at the beginning or end of the story, before or after the audience has had a chance to really get stuck in and seen what the plot has to offer them. One way to really yank the reader out of their concentration and investment in the story is to continuously interrupt the narrative by inserting things that do not belong there. You’ve all seen it: (a/n: LOL!!!) – That right there is immediately going to ruin the tone of whatever else is going on. Not to mention, it just looks bad to insert every thought you have about what you’ve just written in brackets, it’d be like if the author was constantly texting the reader in the middle of the story. I know some people are probably just trying to be funny, but unless it’s a comedy or a satire, then it doesn’t work. (Plus, it probably wouldn’t work for a comedy either unless done well. Writing humour is a lot harder than it seems.)
On a similar note, if you need to move forwards or backwards in time, just state how far back or forwards you’re going, or just put in a scene divider. Writing “Ze timejump brought to you by meeee lmao!” makes you look, similarly to interrupting with stupid author’s notes, like an idiot.