1. Promote EARLY. There are a ton of sites that feature free e-books and all of them have unique subscribers who want nothing more than to download your book, but most of them require you to submit your book at least 2 weeks prior to the free day. If you wait until the day your book is free then you'll miss out on a lot of opportunities to get the word out to new readers.
2. Make sure that your Kindle book is perfect. When I converted Black Cow to Kindle format, a lot of strange typographical issues crept in. For example, about 25% of my L's disappeared, leaving me with words like four for flour and fat for flat. I did check formatting but didn't do another proofread after conversion which meant that the fantastic results - nearly 2,000 downloads that I got for Black Cow - were, to a certain extent, wasted because all of those downloaded books were typo ridden. I was able to get Amazon to agree to send out a link for all of those who downloaded the book to re-download a perfected copy, but it will take them another 2 weeks to get to that (if you've got one of the l-less books, a cleaned-up copy should be with you soon - sorry!). When you're getting your work into so many new hands, it's important that this be your best work. Anything less is not only a lost opportunity, but can impact on your reputation.
3. Be prepared to get a few negative reviews (especially if you don't do #2). If 2,000+ new people are downloading your book, your book is bound to get into the hands of a few non-ideal readers. Take it on the chin and keep smiling. Not everyone will like your book and not everyone is your target market. That's just part of the game.
Overall, both books sold copies in the days immediately following the free days. That alone was worth it, as was the opportunity to get the word out and attract a number of new readers. So I'm definitely planning to do it again. In fact, Carolyn and I are having another go with our book of unconventional love poetry Cherished Pulse in time for Valentine's Day. All you do is go to on Jan 11, 12, 13, 14 and 15th and click. This time, we've followed all 3 points above.