We tend to review the situation when things go wrong, but how about when things go right? The "after action review" is a great technique to use when your customers are elated so you can figure out how to do it more often.
Here's how it works:
Step 1: Identify situations where customers are obviously very happy with the service they've received.
Step 2: Determine what you did to contribute to the customer's outstanding experience. (You may also want to take note of any factors that were beyond your direct control, such as a special sale or the customer was in a great mood to begin with.)
Step 3: Decide what you will do to get a similar result that next time you serve a customer in the same situation. The idea is to deliberately repeat what's working rather than leave it to chance.