TL;DR version: go write more books.

One great thing for all writers about self-publishing is that those s-p authors who have been knocking it out of the park are business savvy, and they've tracked various promotional efforts, kept records, compared results with other top writers, and know what for sure doesn't work to increase book sales. They're also not guarding the information jealously in most cases. As it ends up, some of what trade publishing has been doing for decades is nigh-on worthless in the 21st century. Wait, a few of you are saying. Why would some handful of self-published writers you may never have heard of know more than Big Editor at Big house? Simple. Having real-time reporting of sales lets the self-publishing pros see the real-time effects of various promo efforts. They test promotional efforts in isolation, sometimes a group of them run an experiment together, and they record the results. And what they have concluded is useful for all authors to know.

Remember as a writer, you are not at all a typical reader ("but reviews matter to me!" means nothing to Jo Author's bank balance unless you're willing to buy 100 copies of her new book every day out of your love of reviews), and normal non-writing novel readers are who pays a full-time novelist's bills. Also, remember, just because R.J Famousauthor who is making a half a million dollars per year does some of the useless stuff doesn't mean it caused the millions. Maybe his agent or publisher told him to do it, operating from inaccurate assumptions about efficacy of promotions. Maybe the author simply likes doing it.

The financial goal for most writers I know is to achieve the level of writing income where they're netting 125%+ of what they did in their day job, and that's who I'm talking to here, novelists who want that too. That means selling thousands of books per year, year in and year out, without having to put endless effort (beyond writing new books) into making that happen.

So here's what those clever self-published authors have tracked/concluded:

I) What definitely doesn't sell novels

A. Blogs and blog tours. Particularly don't blog about writing craft and writing business issues. If you're not on the best seller lists regularly, few people care what you have to say about those matters. And even if they did care, and you explained said bookisms better than anyone in history ever has, and through some clever SEO optimization, you actually get strangers to find that post via Google, it still won't result in novel sales for you. If you don't have a fan base yet, there's no reason at all to blog.

B. Social media daily/weekly activity. I know agents/publishers demand you are active on it, but you won't get more than a handful of sales this way. And though it's not "fair," you'd better be young, thin, beautiful, witty and charming to achieve even twenty sales per year via social media—and then you'll get creepy personal messages much more often than you'll get book sales.

C. Book promotional videos. A total waste of time, money, and effort.

D. Articles published in major newspapers about the author. When this happens, authors who can track the results see a blip of about 2-10 extra sales, but at a royalty of $2 per ebook or hardcover, that's much closer to zero income than to a livable wage. Even a mention on a major network TV show only results in 100 or so sales. (someone here had that happen and confirmed this result earlier this year.) Local TV news? It's no better than a big daily newspaper. Publicity as practiced in the 1960s is not worth the time spent on it. Thank a newspaper for wanting to interview you, decline with regrets, and get back to work.

E. Reviews. While an average rating of 1.5 out of 5 stars will likely hurt your sales, most readers do not care about reviews. Most don't read them. Some only read them after they've bought and read the book. So quit obsessing about reviews. Reviews are the result of sales, not the cause of them. Quit manipulating or outright scamming to get reviews—you are risking your reputation (and more) in doing that, for a result that doesn't work to sell books anyway. Let them happen as they will, in their own time, and forget about them, because they are not intended for you.

F. Book tours/signings/conventions. If you like schmoozing with your fans, if you're great at it and can handle awkward people with aplomb, do it occasionally as a treat to yourself, and have fun. But you won't win new fans this way. Veritably no one goes to a book signing of an author they never heard of. You'll probably lose days, if not weeks, of writing time in this effort, and for what? Five sales at each venue? Phht.

G. Paid-for "professional" book reviews like Publisher's Weekly. Repeat after me: Readers. Do. Not. Care. So you shouldn't either.

H. Book giveaways.

I. Do-dads like mugs, T-shirts, bookmarks, and other such items.

J. Great writing. Oh dear, I know this is hard to hear for some of you, and it certainly was hard for me to learn, but you only need to be competent at the line level and slightly better than average at story-telling to be a full-time novelist. There are tens of thousands of beautifully-written novels that never sold a copy. Readers are not agents or critique circles or writers who read the articles that you have read about the craft. They're readers, and they like what they like, and an absence of adverbs and "filtering" phrases doesn't even register with them. If you're trying to promote your prose to an agent, you may need to worry about that sort of thing to get over that hurdle. But to appeal to readers (and if you are either trade or self-published, you will eventually need to do that), you need only be a great story-teller who can twang the heartstrings of readers, writing in simple and clear prose.

J. Family and friends and co-forum members buying your book. This won't support a career and its effect will only last for a few days. So quit bugging your friends and family to buy your book. That's not their jobs, and it won't help you be a pro. It's rude of you to insist or to get huffy over their not doing so. Be a better friend/sister than that.

II) What might work to sell your book. This isn't from the pros collecting the above information. It's from me—what I've seen over many decades of watching and asking and listening and tracking sales.

A. A great description of your book, particularly one with a clever high concept, clearly stated, that makes thousands of strangers say, "I have to read that!"

B. A great cover.

C. Certain kinds of advertising. The two I'm sure can work, that I have seen work for many people at the level of sales I'm talking about (not 25 sales once in a while, but thousands of books in a month), are Bookbub featured deal ads on a first-in-series book lowered temporarily to a $.99 price, and Facebook pay-per-click ads (though unless you are very lucky at first in your design and tag line on that ad, you'll have to keep trying various designs to find one that works, and it costs money to run multiple tests). But when you do the accounting on if an ad worked for you or not, think about the time you spent fiddling with the ads, learning what works, paying for art or worse learning how to do the art yourself, and interfacing with the system—and, if over a year you lost a whole novel's worth of writing time while learning/placing several ads, you have to ask, was it worth it? Or would having one more novel earn you more money than the ads? Also, if you're spending $50,000/year on ads at Facebook to gross $100,000/year (I know a lot of people who do exactly this), maybe don't run ads next year and see if you net that same $50,000. If you can keep your net the same without the ads, why line Zuck's pockets on the way there?

D. Having an always-free ebook that begins a long romance series. It has worked for some people in the past, gaining them fans, but it's no guarantee. It seldom works in other genres (I can count the number of times it has on my fingers). Usually, people who want free stuff want only free stuff. Most won't switch to buying—after they have your freebie, they'll just go look elsewhere for free stuff from someone else.

E. Writing more books. My favorite advice to give is "don't bother with (X promotional effort). Just write more books." This is also not a guarantee, but doing so maximizes your chances of writing The Book that breaks you out, that gets you the first agent or better agent if that's your goal, that appeals to readers and makes them buy your whole back catalog, or that pays your bills for five years or more after its publication date. Just because this latest book of yours (that you're beating like the proverbial dead horse with every form of promotion known) isn't taking off in a gallop of sales doesn't mean the next book also won't take off. The next one might in fact take off on its own. So write the next book. And write the one after that. And so on until you achieve your income goals.

F. Making sure your loyal fans know about that new book. A mailing list is one way that works today (though this will probably change at some point in the future.) So if you have more than 25 fans, maintain a mailing list. (Not your publisher's. Not Bookbub or Amazon's. One you personally control.) Don't use it to chit-chat about your cat's chronic fur ball problem, about how hard writing is, or to brag on the kiddies' accomplishments at middle school. Use it as a precise sales tool, to announce new book releases.

III) What definitely will work

A) Word of mouth. Readers (thousands of them, not coached, just deciding to on their own) telling other readers, family members, and friends about your book.

It's golden. You cannot control it or force it. It happens if you're lucky, and you're thrilled when it does. Because you can't do anything to force the process, get back to writing that next book instead.

B) Having high schools in the US decide your book is required reading for the next couple of decades. Oh, may that happen to you! But you can't control this either.

C) Hollywood options your book, makes an expensive movie of it with big stars, and the movie is a hit. Once again, not in your control. If it happens, congrats! The sales effect is shockingly brief, so keep writing the next book.

Conclusion: the three things that absolutely work, 100% guaranteed, you have no control over. If you are trade published, the only thing of the Part II "might" list that you can control is to write more books. If you are self-published, get your cover, description, and pricing in order, and make sure you are writing in a popular sub-genre, and if you can get a Bookbub, get one, and if you have released two complete series, give FB ads a go, but spend most of your time and mental energy in writing the next book because, once again, you can actually control if your next book gets written.

Guard your time like it's the precious commodity it in fact is. Spend most of it writing books, 1% of it on promotional efforts that might work, and none of it on the promotional efforts that do not work.