So in essence, you're asking us how to talk to the cashiers and clerks at places you frequent?
It should be simple, but I'll try to outline it:
Just be friendly and interested. Ask them questions about their life, and remember what they tell you so you can talk about it later, if the need arises.
By the way, be geniunely friendly and interested, and most of them will be friendly and interested right back, and even ask about your life. Those who aren't friendly back to you aren't going to friendly anyway. Even if they aren't, don't stop being friendly to them. Be the better man. Even if they don't show it, it still makes a difference.
You never know how it may help you later.
It might even help you start a relationship with one of the staff at a place you frequent, if you're so inclined. It's something that I've never tried, but it stands to reason. Chances are, if you hit on someone, they'll talk about it with others they know. If you can influence the others into thinking you're great, they tell your target the same thing, which helps you.
Sometimes, staff may be paid to be "friendly" to you. I don't know about you, but I can usually tell when it's genuine, and it makes a world of difference. Just be nice to them, and they'll be friendly, not "friendly."
One last thing: Don't discriminate. Be friendly to all staff you encounter. It makes their day better, and makes your day better. Even if you aren't the least bit interested in having a relationship with them, they might make a difference. It doesn't make a difference if they're fat, use piercings, old, wrong race (say they're black and you only date white women, or vice versa), or even if you know a dirty little secret about them (don't tell the secret!). It's likely that if your target works with this person, this person will know if you hit on your target. It helps if they like you.
And in any case, just being around people who are friendly to you makes your day better.
Just my $.02,
Ben