1. Give value for money. By doing so, people will be pleasantly surprised by what they're receiving for what they've paid.
2. Be responsive and nimble. Respond to emails, phone calls, and any other questions - promptly and politely. Be prepared to answer questions on every facet of your book, everything you did and why. Make deliveries if practicable, even to passing trains (this is the fun part).
3. Include more information and photos than you think you should. See also 1. It's very unlikely anyone will tell you there's toooo much good stuff in your book.
4. Think about the logisitics involved: shipping methods, marketing and pricing ahead of time. Never increase costs or prices once set or quoted.
5. Remember the "10% writing, 90% marketing" mantra. This kicks in right around the time your book is released. The writing is very important, but marketing becomes very, very important if you want anyone to know about this book you've just spent time creating.
6. Customers and readers really want you to succeed. If they're happy, they'll tell someone.
(I suppose if they're not happy, they'll tell ten people. Better to keep them happy.)