I'm usually flattered and amused when someone flirts with me, unless they are creepy, in which case I feel kind of repulsed and find myself wishing I hadn't left my pepper spray at home. I have to deal with a lot of flirting at work, and that's the one place where I really can't flirt back even if I wanted to due to needing to act "professional." If it's co-workers flirting, I'll usually just give them the, "Yeah, you wish!" smirk, or sometimes a wink. If it's a customer, then I'll act flattered and smile, and then try to get the talk back to the business at hand.