Last summer, my neighbor recommended a particular lawn care business to me several times. He was so enthusiastic about it that I finally asked him one day (during one of those "over-the-fence" style conversations) why exactly he loved that business so much.
"Well," he said, "the first time we hired him to do anything for us, we did it mostly because we didn't have much money to spend. We figured he would do a very basic seeding for us and that would be it, which is what his package promised. Instead, when we got home, we found that he hadn't just seeded it. He had also dethatched the entire lawn for us before seeding it and used a very nice kind of seed that he left information about for us. He couldn't have earned minimum wage for all the work he did, but he said he did it this way because he wanted future work on our lawn to be easier and that us having a beautiful lawn was a good ad for his business. We were so blown away we've never hired anyone else."
This was one of the best examples I'd ever seen of under-promising and then over-delivering. By simply taking some pride in his work and not focusing on maximizing his income on the first visit, he made lifetime loyal customers and evangelists for his lawn care business.
This same approach can apply to almost any business. When you acquire a new customer, simply promise the minimum, but then strive to provide the maximum, particularly in that first interaction with the customer. If you provide tremendous value for them, you'll not only find them becoming a loyal customer, you also will find them sometimes evangelizing for you - a true double win.
Run a computer consulting business? Do some free tune-up work on their computer to make it run faster and more smoothly once you fix the problem they hired you for.
Run an auto shop? Top off their fluids, check their oil, and do other basic maintenance every time there's a car in your shop.
Run a photography business? Put a few extra prints in their final package.
There are ideas like this that work for every business. Going a bit further than what you promised will only take you a bit of extra time, but it goes a long way towards turning an average customer interaction, in which you may or may not get repeat business, into a stellar one where the customer wants to return to your business and recommend you to friends.
Should you advertise such perks? Some businesses do this to get the customer in the door in the first place, but then they fail to over-deliver to the customer. Instead, they often under-deliver and quite often find themselves without the repeat business they so often desire. I recommend not advertising or even mentioning such perks unless you absolutely must have a short-term customer boost. It will be much more beneficial over the long term to under-promise and over-deliver.
What can your business do to over-deliver to the customer without breaking the bank? Chew on that question for a while. You might just find the key to creating more and more happy, repeat customers.