Wrap the contents so they don't get wet or damaged. Many people use plastic bags (such as 1 mil poly bags from ULine) too. They are inexpensive and present a much nicer look than a plastic bag left over from the grocery store.
USe cardboard to keep books stiff.
Packing your book in a bubble-wrap envelope usually works quite well...
But now always. This bubble-wrapped book was starting to work its way out of this envelope when it arrived. Typically, books that have some "sliding room" in the envelope are the ones that tend to destroy the envelopes. If your book doesn't fit tightly in the envelope, consider folding over the top of the envelope to make the envelope smaller.
This is how Amazon sends us our "Vine" books for review. We've never had a problem with these books. They always arrive in perfect condition. But this may not be a viable solution for the one book author. So what should you do?
Here's what one author did to protect her book. A perfect mailer. Note the delivery confirmation sticker. Another good idea.