when serving her just make sure youre polite, like ask "how are you today?" "hows your day been?" "have a gd wkend?"(that last one you should only use on a monday, mainly in the morning but can be in afternoon) and ask it with a genuine smile on your face while looking in her eyes. make sure that every time she's in the shop you're stood up straight (good posture does make a difference!) and look like you're having a good time.... you want to be this mysterious guy who must have a great, fun life that he's walking around the store happy as can be. it might be a bit too forced to be all like "hi whats youre name?" sat behind a cash register but i suppose it depends on what kinda shop it is. i think a better way of wording it would be to introduce yourself and then ask hers, e.g. "hi im <yourname>, ive noticed you shop here a fair amount but all this time ive never known your name"... hmmm that might osund a bit dodgy i think youll need some more opinions on that but i would try it like that.