I struck up a genuine conversion with a saleswoman at Sephora in Manhattan (genuine enough that she told me about her family's pet conspiracy theories), and as I was going to the register, I asked her name "so that you'll get your commission." She didn't work on commission! She was just nice because she was nice and because I wasn't being a bitch. I don't know whether Sephora employees usually work on commission or not.
But this woman walked me through every product I was curious about: "Oh, no, that Tri-Vectin doesn't do shit for stretch marks. It's okay for facial wrinkles, though." And she gave me some wonderful free samples, good enough for the next month of moisturizing.
So be nice to the Sephora people. I've been an asshole there many times, when I didn't intend to buy anything (go in, try everything on, get made up, spriz with expensive perfume, and go!), but the employees can be wonderfully pragmatic with their advice and--if you're not a bitch--generous with samples. Admittedly, they'll usually bust out their take-home samples once you declare your intention to buy something, but given Sephora's general try-it-here business model, it's hard to feel taken advantage of.
One last thing: girls, go in with ONE goal. Either you're looking for ONE thing to buy, or you're just looking around, because otherwise the sheer display of products can you convinced *like that* that you need forty-five products just to look "natural."