The beauty of this method is that women of such occupation will be very open and receptive to your approach. There is no evident bitch-shield or up-tightness you have to dissolve. These girls are locked into a service-minded, customer-first mindset. If they reject you, report them.
There’s no need to be shy or even guilty with this practice. You’re in it to brush up your game, and they’ll never know your intentions. In fact, most are bored witless at their jobs, and by talking to them, you’re doing them (and their employers) a great favor by making them do something.
If you’re in a clothing store like Zara or H&M, pretend like you’re shopping for a birthday gift for a female model friend of yours (good for social proof). Ask for their recommendations or “what’s in” lately.