Writing is a business that can make you crazy. I say this as someone who has been earning her living writing for nearly twenty years. It's important to point that out, because I was trying to earn a living for about FORTY years. You see my point?
Anyway, as you can imagine, both writing and publishing have changed a lot--A LOT--since I first banged out my first novel on my dad's manual typewriter.
But some things, unfortunately, remain universal. And they are as ineffective today as they ever were. Let's take a look.
1 - Ignoring publisher guidelines. Worse, explaining to editors in detail why you are ignoring their guidelines, why your work transcends their guidelines, why they should never have come up with these guidelines in the first place.
2 - Sending nasty letters to...anyone. Starting with the editor who declined to acquire your work because they didn't believe it was a good match for their list (i.e., they don't think it will sell) to berating reviewers who didn't like your last book. I'm not saying that the editor was right or that the reviewer was right. I'm saying sending hate mail gets you NOTHING and NOWHERE.
(And at the very least, wait to build your bridges before you burn them.)
3 - Writing your own reviews. OH. And writing bad reviews of authors you view as rivals. Just. Don't. It's pathetic. Truly.
4 - Mistaking advertising claims for career advice. For heaven's sake. If someone is making a living selling you HOW TO BE A SUCCESSFUL AUTHOR products rather than being a successful author themselves, you need to view their messaging with a critical eye. I'm not saying the products can't be useful to you or there's no good advice to be had from these folks--I take some of these courses too, and they're full of great advice--but use your common sense. If it's that easy to get rich writing, why aren't these people writing? Why do they prefer selling YOU their Secrets To Success plans?
5 - Writing what you think will sell rather than what you'd love to read. It's not one or the other. Assuming: A - you're writing your own books, and B - you want to write fiction for a living, you have to balance what you love with what will sell. Yes, there will be compromises along the way. That is how commercial art works. Writing is difficult. It is one of the most difficult art forms because it requires such sustained effort. If you don't love what you're writing, then it's just a job--and not one of the better paid ones.
I hope someone out there finds this helpful information. But knowing writers as well as I do, probably not. :-D