Like the majority of authors, those books I have self published are available in both Paperback and Ebook versions from Amazon.
The paperbacks are managed through Createspace, and the ebooks through Kindle Direct Publishing (KDP).
Whilst most are aware of it, some authors still don't know about the Kindle Matchbook scheme. This allows you to set a discounted price for the ebook for anyone who has bought the paperback on Amazon.
Since I started, I had viewed this as an additional income stream. I set the Matchbook price to 99c and got a few conversions.

Recently, however, I've changed to offering the Kindle version free to anyone who has bought the paperback.
Firstly, for some books the paperbacks are in black and white, whilst the version for Kindle Fire is full colour. The ebook version, therefore, adds value to the product. This is particularly true for the Walking Through the Past series of walking guides to sites of archaeological interest.
Secondly, I have included that the ebook is free with the paperback in my advertising. It's even mentioned on the product description on Amazon. This has slightly increased the paperback sales, without reducing the ebook sales.
Thirdly, the more people who have the ebook, with its clickable links in the end matter, the more likely people are to browse my other books.

So, if you haven't already, check out Kindle Matchbook, and see whether you could use it to boost your sales.