In my "day job," I used to write advice articles for freelance writers, and I constantly cautioned them against undervaluing themselves. As a freelance writer, you have to set a minimum price and refuse to waver (unless the economy crashes, but that's a whole different issue). As an indie author, freebies are your bread and butter. You can't possibly undervalue yourself or give enough books away for free. What I'm saying is this: never, ever shy away from giving out freebies.
Homespun wisdom dictates that you should never give anything away for free. Do so, and why would anyone end up buying it?
But that's just not how it works with books; it's never how it has worked with books. What's the most-read, most-sold, most-known book in the United States? The Bible. It's given away for free all the time, and you can find copies of it in every hotel and church pew in the nation. Yet it also sells more copies every single year. Clearly, people will both drink free milk and buy it still. If that wasn't the case, libraries wouldn't exist. They're dedicated to lending books out for free, particularly all the big bestsellers and legendary classics that everyone wants to read. You can go get a Mark Twain book any time, but people still buy all of them all the time.
Under that philosophy, you should give your book away as often as possible. If you could put a copy of your book in every single hotel room across the land, don't you think you'd get more reviews on Amazon? You certainly would. However, this is not a logistically feasible book promotion.
Giving free copies of your book away on your blog...now, that is feasible. It's also easy. There are any number of file-hosting sites that will allow you to upload your book file in any format, or multiple formats, and provide you with a link so you can share this file with anyone. I'm partial to Dropbox, but you should explore what's out there and find out what works best.
Smashwords also allows you to run free book promotions any time you like. They give you the added advantage of choosing to mark your books down by percentage instead (25% off, 50% off, and so on). However, Smashwords requires users to have both an account and a special code to take advantage of promotions, and that can be a hassle.
Amazon's KDP program doesn't just allow indies to give their books away for free, it requires it. You choose free days every month as a matter of using the program. I do not advocate this program for indies, however, because Amazon KDP requires an exclusive listing.
Giving books away for free exposes more readers to your books. Word of mouth is still the most effective book marketing tool in the world, and it's not something you can buy with money. You can buy it with free books. Give them away, get readers talking, and their friends will buy your books when the free promotion is over.
Many indies are afraid of freebies because they need the sales income. But you'll actually sell more by doing free book promotions frequently. Exposure is everything in this business...and free is free. There are people who will download and read a free book just because it's there. When they fall in love with it, they'll tell everyone. You can't buy that sort of promotion...you can only give it away for free.