Let me step back first, and say that the single most important element of marketing is exposure, in my opinion, so the best marketing technique is whatever gets you the most exposure for your book. Simple, but true.
It may be different for each author, and each book. If you have a cookbook, for example, your best exposure will be to foodies – writers and bloggers and cooks – and this may come via a social media outreach. If you have a children’s alphabet book you want to target libraries and schools, and of course parents, so your focus will be different but your aim – of reaching your audience – will be the same. The common denominator here is the author who knows their material and who sees their target audience. Therefore the best marketing tool is YOU! Without you, the book won’t sell. You can hire a marketing firm (hey, I am standing by J), but ultimately, you the author must be at the ready to push your work forward and seek out your readers.
Again, this can be different for each author and each book. Love social media? Then use it: write a blog, tweet, develop a following, have a contest, post interesting content… Not a fan? Then do what you are comfortable doing: go to schools and give a talk, do a reading and signing. Use your best marketing tool, YOU, to represent you book and find your audience!