My manager has offered me a little promotion, but I need to over come one little obstacle. Help!?

I work in a very small expensive retail store, there are currently 6 girls employed, my manager has decided to bump me up, more hours and a possible pay raise. But she said that she noticed Im too nice with the other girls and I let them take my sales...I do, cause I dont want to be mean, and the girls dont do it on purpose. Which brings me to my question, how can I work with my customer, what should I say to her when she tries on that dress, should I strike up a conversation? And about what? What will keep my customer with me?