I get called sir, I have pretty much ever since I started buying things on my own.
Just keep calling them sir. There's not really much else proper to say if they're a customer. Whenever I'm serving someone I call them sir, because there's nothing else to call them. Sir and ma'am.
Now, I do live in Texas, but I experience this everywhere I go.
But yes, if you don't like it, try to avoid it. However sometimes you have to get someone's attention without knowing their name. In that situation you have two choices:
Formal - "sir!" "ma'am, excuse me"
Informal - "Hey you!" "Hey kid!" etc
When you are addressing customers you kind of have to use formal no matter what. When you're just talking to a stranger you use formal if they appear to be socially superior, older, richer, or more educated or something. You can only really use informal if you yourself are in a powerful position compared to the stranger, which is rare unless they're a kid.
Even then you can try to avoid using the informal address by just saying: "Hey there! Hey, excuse me!" Failing that you can get another stranger to help you get there attention. The person who notices you, you look at him or her and say, "can you help me get his/her attention? *points*"
I dunno.
It does depend where you work though I guess. If all your customers are college kids and you're one yourself you could get away with "hey how's it goin man" right off the bat.
I love thinking about this social shit. You realize you know it without having to even think about it. Human beings are amazing creatures.