A couple more ideas

[detail man]

What I have done over the years and now our custromers look forward to it is taking a picture of each and every vehicle. Once we take the picture we blow it up to legal postcard size and we write a hand writen thank you note on the back and send it to them. Most our customers then will put them on their refridgerators, facebook and even hang them in their office. Another idea is get a phone number people can remember. example our number is 855-BUFF probably the best thing I ever did

[S2K]

That is great customer service! Most businesses today totally forget who you are after they get your money. That extra step of remembering me would mean a lot.

[Bunky]

Interesting idea.

[Porsche Pilot]

I do the photo card thing also. My clients love it! A little extra effort and expense goes a long way into building lasting business relationships.