We are inundated every day with email, status updates, tweets, texts and notifications...all of them digital. When was the last time you got an honest to goodness letter in the mail? Personally, I can't remember.
When the time comes, consider sending out a personal letter to your friends and family members. Let them know about your book, ask them to share in your excitement and thank them for the support they showed during the process. If you can, include a coupon code or offer to send them a free copy of the book. Don't make this a sales letter. We get enough of those in the mail. We call them junk mail. Don't create junk mail.
And you don't have to stop there. If you have an event planned, consider sending an actual letter out to local libraries and the media. Or consider a handwritten thank you to anyone who's been a part of getting your book out there.
Obviously, this takes more time and isn't always going to be appropriate, but an actual piece of mail can have the effect of standing out in today's tech heavy world. The recipient knows you took more than a few minutes to fire off an email and that can only work in your favor.