In my opinion, the key to good social media marketing is not self-promotion, it's promoting others. Not only is it a nice thing to do, promoting others will pay off for you. It's not a new concept, but if you do good things for others, they are more likely to do good things for you. Even if they don't turn around and promote you, they'll remember your name. You're building your brand and building your network. Don't ask or expect people to turn around and do nice things for you if you do nice things for them, but it will happen organically if you help them genuinely - without expecting anything specific in return.
So start your campaign of niceness, here are some ideas of things you can do for other authors:
1) Review their book. This is number one for a reason. Yeah it's actually even more important than #2. Studies have shown that consumers pay more attention to customer reviews than almost anything else. Mid-list and independent authors are especially reliant on this free but effective form of advertising. After you read a book that you enjoyed, go ahead and review it. It doesn't have to be the most well-written review ever, just give your opinion. Then make sure and post that review on Amazon, B&N, and Goodreads.
2) Buy their book. This one is pretty self-explanatory, but important. Authors do really appreciate this. :)
3) Offer to interview them for your blog. Stephen King and Stephanie Meyer may not be available, but 99% of authors are not bestsellers. A good amount of mid-list, debut, and independent authors would be very appreciative of your offer, so it doesn't hurt to ask. Put yourself in their shoes. Somebody likes your books and is interested in you. It's free exposure and short blog interviews don't take very long. You'd probably say yes to as many as you could.
4) Promote their book on social media (and in other human interaction). Books rely heavily on word-of-mouth advertising. Even the big name books aren't sold like films or new cars. There are few commercials or billboards about them. But how many times have you asked a friend for a recommendation? So recommend the books you read! Amazon makes it really easy to share links on social media. And of course, don't forget to talk to your real live human friends.
5) Create lists. Any user can create lists on Amazon "Listmania" or Goodreads "Listopia". You can call it anything you like, such as, "Best YA Paranormal Romance Books of 2012." And then....when people do a Google search for "Best YA Paranormal Romance Books of 2012" they may actually find your list. Bam. You're an automatic expert.
6) Tag their book's page on Amazon. Authors and publishers can't do this themselves, so it's up to you! You will tag the book with search terms that you want the book to come up under. They might be things like "YA" "fantasy" "sci-fi" "zombies", etc. You can also vote up helpful tags that others have entered.
7) Like their book's page on Facebook. Yes, sort of an obvious one. But, yes, authors do like this.
8) Sign up for Amazon Associates. This one seems a little backwards, but hear me out. In my opinion, Amazon Associates is a great way to monetize your blog. Basically, you get money when you link to Amazon products (in our case, probably books) on your blog or social media. Chances are, you post about books anyway and include links, so this just gets you a little money for something you do anyway. How does this help the author, you ask? Trust me, when you know you might get paid for linking, tweeting, or posting book links to Facebook, you are a lot more likely to do it.
9) Like their book's page on Amazon. This one takes all of one second. Just push "like".
10) Vote on "helpful" reviews on Amazon. The reviews that are voted most helpful appear near the top. If you like the book, then we can assume that the "helpful" reviews are the positive ones. :)