As a server, you are depending on your customers tips for your income. This is hard logic for some to understand. (See Steve Buschemi in Reservoir Dogs.) Often, customers see servers through a haze of suspicion. At best, viewing their every move as some attempt to increase their tip. At worst, they fear we have poisoned them. (See Fight Club.) Sure, I've known some mercenary, customer-hating servers in my day, but most, at the very least, just want you to enjoy your meal.
An excellent way to calm a customer, endear them to you, and help insure they return to give you their business is generosity. Buy them a drink. Whatever they are drinking, or a cup of coffee at the end of the meal. Buy their appetizer and start your relationship off with a sign of goodwill. Buy them a dessert and drop it at the table with a, "Thanks for dining with us, I hope you enjoy this pie. On me." Leaving them with a positive impression at the end of the meal.
Of course, there are restrictions depending on what kind of boss you have. Even if your restaurant has a strict policy against this kind of behavior you can usually find a way to do it. There's a couple things to remember: 1. As a server, you earn the bulk of your income from the tips your customers leave, making them your true "boss." Help them feel that way. 2. Your boss should always be reminded that return business is the best business. Spoiled customers come back, and they tell their friends about your generosity.
There are other ways to exercise generosity. Take your time at the table, indulging them in their questions. Refill drinks without being asked. Welcome them when they arrive. Thank them often for dining with you. Get creative and come up with some on your own.
There are many options, but I think the best way is through free stuff.
That's a "thank you" everyone understands.