Whether you are trying to sell books 10 years ago (like I was) in a brick and mortar store, shaking hands, schmoozing store reps, trying to get a write up in the local paper, etc. OR whether you are doing it today trying to get followers on Twitter, FB, getting friends on GoodReads or doing a "blog tour" (seriously, how things have changed) - the key, number one thing I have found is:
YOU HAVE TO BE A PART OF THE WRITING/BOOK COMMUNITY AND WANT TO SHARE SOMETHING THAT WILL LAST!
You can't just roll into a blog/forum/post and say "yo, check out my new book, it rocks, tell your friends" and you can't just mass email all 600+ of your FB friends and say "finally, after months of hard work my new book is out, it's only $2.99 on kindle, go get it" - you will be vastly disappointed!
Don't write a book/story to sell it - write a story to add something to somebody's life; whether it's humor, intrigue, hope, or just entertainment. But write a good story, nay, write a great story that will want people to keep turning the pages. Then, as part of a writing community (whether in real life or online - I would suggest both) people will know that you have finished it, people will want to read it, and then, because they know you aren't just trying to make a quick buck and force it down their throats, they will share it, the sales will come, and you will have enriched others.
And (even more importantly) because you are a WRITER, and not just a guy/girl trying to publish a book, you will have continued to write and become better and have more stories to share, and over time they will keep coming back, wondering what you are working on and you will continue to be able to give and give and give.
That's how I see it anyway...it's a beautiful thing - the legacy that a writer can leave. We really can make a difference for lots and lots of people!