One marketing tactic is to have at least two to three e-Books available and offer one of them for free over a longer period (a few months or years). If your free e-Book is interesting, readers will buy the other e-Books, for which they have to pay.
Even stronger way is to offer one novel for free and one or more short stories. You have more chances that readers will first choose free short story (because they don’t have time) and if they will like it, they will upload also free novel.
However, additional promotion on social networks is also necessary, otherwise between 300 and 1000 readers will download your book, but this will not be of much help to you.
The number of free e-Books on offer is enormous and readers have consequently become choosy even here. They will not decide that easily to spend a few days reading your e-Book, even if it is free. So you have to persuade them to do so. And free short (10 minutes) story is a good way to do it.
Another fact you must take into account when offering a e-Book for free is that from the moment you publish the book on Smashwords, at least 2 to 3 weeks will pass before the e-Book appears on other bookselling portals, such as Apple, Barnes & Noble, Sony, Diesel, etc. I made the mistake of waiting only 7 days and because the book was not selling, I put the price back to 0.99$. There are many examples when a e-Book sells only a few copies a month, and then sales suddenly grow rapidly. Amazon’s policy is not to sell e-Books for free, but if you will offer your e-Book for $0.00 on other portals, Amazon will soon (after 10 days) change their price to $0.00 too.