My two cents from the research I've been doing of self publishing for the past year, I know I'm making these numbers up, these are just arbitrary estimates according to my experience, but your book cover is 60% of your marketing, 20% is from keywords/search engine, and the rest is you going out to make a concerted effort to get your book out there.

This is one of the self publishing success stories I've been following since around it's release. I bought the book when it was just at like in the top 100 000 ranking of Kindle books sold. Right now it's around the 4000s, and I'v seen it as high in the thousands.
I first caught wind of it when it was being advertised in a blog's site for best new books.
The first thing that popped out to me about it was its cover.
In today's fast paced economic world people spare little time evaluating on purchases they're looking to buy before they move onto the next thing. They need a reason to keep looking into your book, and the gateway towards that is the cover. People's attention spans are as short as ever. If you don't have their attention with your cover, then they're probably scrolling down looking for another book to capture their attention.
Having a really good cover is ABSOLUTELY IMPORTANT. I get sad when I see hard working authors doing everything in their means to get their book out there, but no one really gives the time of day to give it a chance because the cover doesn't stick out to them.
The second part is keywords.
http://www.thecreativepenn.com/2013/02/28/keywords-metadata-discoverability/That article really opened up my eyes to self publishing success. There's nothing I can add that isn't already said in that article, I highly recommend that everyone reads it.
I've signed up for Merchant Words that do word searches for you on what people type in amazon so that you have better search results. The $30 a month fee is pretty hefty, so I don't suggest getting it until you got the ball rolling. All of my books saw better sales when I signed up for it though.
I did an experiment self publishing a short story (although it's 120 pages long so it's not even short) that I wrote when I was 15 under a pseudo name with no prior book releases to it (of course it went under heavy revisions and editing). I was looking over stock photos that fit my story on a book designer website and found one for $30 that looked professional, something that Barnes and Nobles would put up on their shelves. I wanted to see how powerful holiday sales can be without me having to advertise the book to sort of test out my theory above, because I heard from another author she got the most sales on Christmas because that's when people are on break for the holidays and are looking for something to sit down and read. And also because people get their brand new kindles and are eager to make their first purchase.
However my mishap was I mistook "the holidays" for just Christmas, and I released my book way too close to Christmas. Christmas is the most competitive time of the year, and my book didn't stand out from books that were released earlier in December that gained traction throughout the month leading to Christmas. I've noticed that a lot of books get released early to mid December, that would have been a much better date for me to publish it.
Nonetheless since I published that short story I sold 20 books and it's been borrowed/read on KU 30 times. And that's without me not lifting a finger as far as spreading the word about it goes. If I had released it earlier in December like everyone else I imagined I'd see far more success.
Hope that helped everyone.