Exceptional service is rare, so when I receive it I usually try to let their manager/store owner know. If large co., then I let manager know but also email corporate. In my email I describe the exceptional service I received and I ask that the employee be recognized for it.
I just did this yesterday for a banker at a Wells Fargo branch. He went out of his way and helped me out with a somewhat complicated matter. I asked him how I can help him get recognized for his exceptional service. He was thrilled.