I really enjoyed working retail (bookstore), and if it hadn't been for the owners screwing me over when it came to the management spot opening up, I have even gone back for some part-time work. (They expected me to do EVERYTHING the managers at their other stores did, including interview candidates, arrange schedules, and handle all the shipments, but I didn't get a manager title or pay rate.)
I loved Christmas particularly, because we had a lot of people coming into our small bookstore knowing that "Jenny would love a book," but who had no idea what she'd like. So I got to have them tell me a bit about the person, then basically sell them the book I would have chosen if the gift recipient had come to the store in person. I think because we were a discount bookstore (publisher overstock, so new books but at 50-70% off what you'd normally pay) and we were in one of the small stores in the mall, people were a lot more understanding when we didn't have the newest and best stuff - we just didn't have the space. And yes, I did run in the back and root through boxes on occasion, when I thought we might have another copy of something back there.
So my addition would be "Don't assume everyone is having as bad a day as you are." There were definitely customers who came in expecting to fight - and I "helped" them if I had to, but I wasn't going to approach them to really offer much. Customers who came in and were polite, positive, and open were the ones who got the best attention and service.