So, you're either not in the United States, or simply want to back up the indie bookseller site Smashwords instead of Amazon. That's cool. But, what's the easiest method of getting that .mobi file onto your Kindle?
After you buy the book on Smashwords, you will see a list of the formats at the bottom of that book's page. On the line that states this:
Kindle (.mobi for Kindle devices and Kindle apps)
you will see the Download button on the right side. Click that, and choose to save it to your PC. (I don't have a Mac, but it should be similar).
Now, take a look at your Kindle. Click the menu button, and then Settings. On page 2 of the settings (of a normal Kindle like I have), you'll see the "Send-to-Kindle E-mail" line. They show it as something like firstname.lastname@example.org. However, if you send books there, Amazon may charge you a small "upload" fee. Heck with that! Instead, use email@example.com. It works the same way, but you must have your Kindle connected via a wireless connection to the internet. Most of us do at home, or in a McDonald's, anywhere with a free wireless hotspot, etc. You just can't use 3G, if you have that model for this free transfer.
Next, open up your email program (or Yahoo in a web browser, or Hotmail, etc.). Create a new email, with the To address being the firstname.lastname@example.org address. Now, click Attachment and go to your Downloads folder (or wherever you chose to download that .mobi file from Smashwords). Type in a little subject and body (doesn't matter much what it is, I tend to put the name of the book). Click Send.
Now, the first time you do this, Amazon will reject it, and send you an email warning you that XXXX@XXX is trying to send a book to your Kindle, and do you want to allow it. Click their line to allow it, and then go to the Sent folder of your email and re-send that email. In the future, you won't have to do this.
That's it! It's an easy method to save the .mobi, and then just email it to the Kindle. No connecting cables, dragging files around, etc.
By the way, this also works with the .prc files for Kindle (Amazon felt special, and had to use their own file extension, but they are really .mobi files with a different name).
Hope that helps!
'Till next time.