There's a saying about customer service suggesting that "You're only as good as your last transaction". In reality, you're really judged by your next "transaction", meaning a piece of behavior in the present moment that reflects favorably or not on your restaurant in the eyes of one or more customers.
One word of warning here. Never indulge in a positive piece of action towards a customer that can't be repeated - unless you are prepared to repeat it. An example: a customer is taken unwell during a meal (nothing to do with your business). You order a taxi to take the person home and pay for it on your own tab. People would accept that as exceptional behavior.
However, if a customer asked you to pack up the uneaten portion of their meal so that they could take it home - and you do so - this is then expected action by that person, anyone they mention it to, and all others who were in your restaurant at the time! Failure to do so next time...
The above deals with customer expectations - and it's the stepping beyond these that are a key factor in improving customer retention. Offering great food, matched with stylish and committed service should never be a plus - it should simply be an expectation; a base starting point for restaurant customer service if you like.
It's those extra touches that matter in terms of a positive response. These are the areas that are shared on Facebook or Twitter through positive comments. Often, these might be seemingly unimportant little things - such as remembering names and slivers of information. "Becky not with you tonight, Mrs. Gray? Has she gone back to college already?". "Didn't you enjoy the calamari last time you were here? It's not on the menu, but I know a fresh supply was delivered an hour ago. Do you want me to ask the chef...?"
Of course, the little points will vary greatly; the above might be fine for a small family-run establishment but simply aren't feasible in a busy hotel or at an airport location.
The key is to brainstorm the small things that would find favor with your clientele, and then, if you can, build them into the way you do business. You might even hold a competition for your team to develop their own suggestions. Incidentally, taking such action is really like showing customer service to your own staff by making it clear how much you value them!
Take a look at some of these unique restaurant promotions to give you some ideas. There are some gems here!