There's a few key things you MUST HAVE to sell your book, based on my research of the marketplace and careful chatting with other writers. 1. You must have a great cover. Your readers will be scanning a page full of twenty or so little postage stamp size books. They will then click "next page" unless a book cover & title interest them. That's all you have for advertising - just your cover and title. That is your initial hook. Cruddy covers are generally instant death for ebooks.
2. You must have a great blurb. Learn to write the blurb. Study the blurb. The blurb is not a review - you do not tell the reader "this is the hottest novel since..." or "this is just like so-and-so's books". SHOW, don't tell. ;) Read the backs of some paperback novels, if you're writing a novel. Once your cover hooks someone to the book page, only your blurb will hook them to take the next step. Just like writing a query letter is a different skill from writing a book, writing a blurb is its own skill. Learn it.
3. You must have an outstanding sample. Ditch as much front matter as you can. Shrink your image to take up as little file space as you can. Only the first X% of an ebook is in the sample, and it's by file size, so cover seems to count (please correct me if I'm wrong there, that's my observation, concurred with by a few other Kindle authors). Small file size JPG for the cover (reduce quality a little in Photoshop when saving), put as much front matter at the back of the book as you can, and give them the biggest sample you can. The sample must be interesting. The sample must be great writing. The sample must grab the reader so well that they immediately buy the book when finishing the sample.
Cover to blurb to sample to purchase.
4. You must have a good price. That doesn't mean 99 cents, unless it's a short story or a loss-leader story, say the first of a trilogy. I'd stay in the "money range" for most books, which means $2.99 and up. No higher than $6, I think, since I've noticed sales seem to take a hit in that range for many writers. Keep it high enough to make a good profit on sales, and low enough that it's *easy* for the reader to click that "Buy" button after finishing the sample. You don't want to make them think about it. You want them to click. ;)
5. You must have a compelling book. The rest of the book has to be as good as the sample. Remember, they can return the book if the sample is stunning and the rest of the book is your grocery list. ;) You want them to walk away from reading with a feeling of deep satisfaction in the purchase.
6. You must write other books. If you want to sell, and keep selling, you should be writing more books. Your next book is your best form of marketing. The more books you have up, the more each tends to sell. Short stories are great ads for your other writing, too, but books are best. Continuing to write new books keeps recent releases of yours in the spotlight - in an ideal world, you want to produce at least one every three months, to stay in the "released in the last 90 days" category. Realistically, most writers can't do that: jobs, family, and lack of experience stand in the way. So we do what we can. Produce quality each time, but continue producing. I suspect - but can't prove - that ebook publishing is a bit like the old shark swimming myth; if you stop moving, you're dead.
Every book you have up compounds the chances that a reader will see the cover, like the blurb, download the sample, buy the book, have a great reading experience - and then go buy another book you wrote. Get them to buy a few of your books in a row, and you have a fan. That reader will remember your name, maybe for years. They will look for your new releases, and grab them. If you have just one book up, your name will likely be forgotten shortly after the book is done. You need to reinforce the brand recognition by getting readers to read more than one book.
Some folks like Bob Mayer and John Locke have even gone so far as to suggest you not begin doing any marketing - or even not bother publishing at all! - until you have 3-5 books ready to roll. Lots of indie writers burn themselves out marketing away, spending hours on that process which they could be using to write the next book. Which remember, is the primary form of marketing for all your work. ;) Stay on task, keep getting your books out and up and rolling. I favor the publish right away, and publicize later option. It's not like the books are going to vanish if you don't market them. They'll still be there a few months later. Get the other books up, then market the lot of them. With three books up, every tweet, every blog post, every ad, every review becomes three times as effective. With five books, it's even better.
Even if you do all these things, it doesn't guarantee success. But failing to do these things invites failure.