No one should have to kiss my ass just because I'm buying groceries from them. It makes me uncomfortable when service personnel older than I am call me "Sir". I particularly don't require the overly familiar attitude adopted by Safeway clerks, just because I entered my Club Card number and they read my last name on the display.
So here's what I've begun doing:
Clerk: Thank you, Mr. Keane.
Me: Please. Mr. Keane is my father's name. Call me Sean.