1. Generally speaking, indie authors benefit most from networking. This is something that I've taken to heart and it's great to make friends with other authors and promote their books because they'll be inclined to promote yours. Authors who worry about their own success alone generally fail in this business. Authorship is not competing for readers because when someone finishes Anne Rice, they're more inclined to pick up another vampire novel in the same "vein."
2. Diversity is the key when attempting to get your books to the largest audience possible. The Rules of Supervillainy is my biggest success so far and it's a huge success for a mainstream publication instead of just an indie press. Part of that was the fact it managed to hit a niche in genre fiction but also because it reached a huge audience of audiobook listeners versus the somewhat overloaded ebook market. Try to make sure your books are available in multiple formats of audiobook, ebook, and paperback.
3. A lengthy portfolio is pretty much a necessity in the modern ebook marketing world. You probably won't make your fortune on one or two books but will benefit most from having a robust series of novels with multiple complete series. The fact Amazon and the internet means books "stay" in print means that you probably will get a trickle from each of your series as long as you've promoted them enough for people to want to check them out in the first place. In short, don't expect to be a full time writer after your first book, expect it after your thirtieth.
4. Contradicting number 3# to an extent is the fact you need to manage your release schedule. Releasing a bunch of stuff at the same time or in rapid succession means the people you actually have managed to win over with your writing may be overwhelmed. After a release, you should spend time promoting your new release and make sure that people know about it.
5. Methods of promotion include Twitter, Facebook, blogs, Reddit, and other social media presence. Generally, no one cares about spam and if you show up to mention your work is now available for 5.99 on a group then don't be surprised if the reaction is a resounding "meh." Engaging with fans and making yourself interesting enough they actually check out your work is a big deal. Basically, making yourself a presence in your genre's community is an important part of success.
6. Managing expectations is something that a lot of authors have difficulty with. Gone are the days of which you can just depend on a big publisher to release your books to all the bookstores in America and assume you'll release in the 10s of thousands (and lose 99% of each sale to your publishers). Instead, a 1000 copies is an extremely good lifetime sale for an indie title and sometimes takes a while to take off. Success also breeds success as you can expect sales to pick up when one book succeeds and develops a fandom who buys your others.
7. Beware of scams. I feel like this is something that is unfortunately endemic to the indie scene but there's no end of people who want to charge you $100 to publish your book on their mailing list of people who don't actually read the books. There's also plenty of scam publishers who will take every dollar you have and then go bankrupt. Beware, too, of editors who give it a spellcheck and charge $400. Check out sites like Writer Water Cooler and get help from your fellow authors to avoid this.
8. Understand that financial success in writing is an ongoing process, which is a polite way of reiterating the earlier point that becoming a full-time writer is a long process. You'll probably not be able to quit your day job for a good long time if ever. Do it for the art first and the supplementary income second unless you're one of those people who knows how to write porn for $10,000 a month. No, I don't know how they do it.
9. Engage with the community projects of fans like the Self-Published Fantasy Blog Off, ABR awards, interviews, blogging, and guest posts. It's not going to be an automatic boost to your book and plenty of blogs, like mine, have like 20 people who follow them but every bit counts.
10. Reviews provide the illusion of success, which can sometimes lead to real success. Lots of people don't trust independent books with 5 reviews or less. Instead, they tend to view books with 60-200 as things which are successful or worth reading. This requires the equivalent of MMORPG grinding to get people to check them out. Amazon, Audible, Goodreads, and blogs all help.
11. Try and have multiple installments of a series in order to get people who like the original work to pick up the next. If you can develop a fanbase then it's good to have people who want to buy the next novel. This is something of a risk because there were always be diminishing returns. The first book will always have more books sold than the second and the third with some people dropping the series. It's best to keep a 3-5 book series and any subsequent novels in your world to be spin-offs.
12. Don't go chasing trends. There's always someone seeking the "next big thing" and that's just a recipe for disaster because yes, sometimes the market will be over-saturated and sometimes you'll luck out by being at the ground floor of something awesome. Superhero fiction was niche enough for me to be one of the people someone looking for "superhero fiction" found when they entered it into a search engine but it also was a massive genre to begin with. Write, instead, the books you're inspired to write because a wide pool often is deep in its fanbase.
13. Remember a book is and always will be judged by its cover. Something eye-catching and memorable will always be more likely to get you picked up than something which isn't. Commissioning art is an expensive investment but something that is worth investing in if you can afford it. Pre-rendered art are a good investment if you look around but beware you'll probably see someone else using it. I've encountered 3 series that use my Agent G model after all and that's just people who have brought him up.
14. To directly contradict my earlier point about trends, there's something to be said for the power of public domain. The Cthulhu Mythos, Sherlock Holmes, King Arthur, fairy tales, various Pulp heroes, Alice in Wonderland, and more come with their own fanbases. If you are already a fan of something, you can potentially jump on the bandwagon. Don't expect your dark and sexy interpretation of the Wizard of Oz to be an automatic best seller but it's not like there isn't a market for it.
15. I cannot stress this enough but make sure you have a proper editor and don't be surprised when mistakes slip through. Being an independent author means that you can probably fix every single one of your problems before your next book sale goes out but there's no getting back your first impressions. Make sure you identify your flaws grammatically and work to correct them, follow the standard editing format, and always incorporate corrections you've identified. Mind you, no one actually cares if you have a few commas off, but readers (as well as fellow authors) can make mountains from molehills. Also, don't panic about the mistakes either as many readers don't care as long as it's legible--don't panic but do fix.
And some bonus advice:
Remember that you're not actually competing with your fellow authors. When someone finishes reading your book, you're not losing a reader when they pick up another one. Review other author's works and promote them. The good will you get from directing readers to work you like will reflect back on you and fellow authors will often return the favor.