The only thing that I can suggest is to treat it no differently than any other shopping experience; go in there having a good idea of what you would like to see and just be your natural self. Wear what you normally would, if that's sandals and jeans then wear sandals and jeans, and of course be friendly/down to earth with the SAs since I'm sure that they put up with enough faux-aires and attitudes during their shift.
Yes, if you're purchasing a bag then the SA will automatically slip it into a dustbag, wrap it in tissue paper, and then box it up for you (don't hesitate to request a ribbon, just in case, if you're a sucker for those types of things
). Your receipt will be printed while your bag is being packaged up and then placed into an envelope which the the SA will then come around the counter to hand to you with both hands. When you're all set to leave then the SA will walk you to the door, pass you your new treasure, thank you, and wish you a pleasant day.