There are hundreds of ways to ask for people's feedback but the most immediate I came across recently was at Heathrow airport. It's a machine that is positioned directly after the security check. Only one question is asked and the customer can choose between four levels of satisfaction.
I love this straightforward approach because it's fast, direct and easy, particularly for non-English speakers. I am sure it gives the operators a fairly good idea of the overall level of satisfaction or dissatisfaction of their customers without having to offer money or prizes in return.
As a customer I would have loved to see the results immediately but that could be asking for trouble. However it may well be something we see in the future... CF