An opener I have been using lately is to Open Shop assistants is " I am shopping for my sister/mother for Xmas/birthday party Bar Mitzvah Whatever." Can you help me find something. Instantly I have a topic. I usually get them to try the clothes on as my mom/sister is always the same height as this assistant. It builds rapport well. However I have been struggling to number close because I dont buy anything and before you suggest Me buying something remember I try this in every shop at the Mall. How do I number close from here, I've had some limited success with " I wish you were always talking to me when I shop for my family" but need something more concrete.
Ideas/thoughts

I'm fairly new to this, but I'll give my opinion and I'm sure someone will tell you if I'm off my rocker. All I can say is that I've given advice similar to this in the past to friends, and the situation usually worked out like they wanted it to.

First of all, I wouldn't buy anything just for the sake of getting her number. I've done this in the past and the only thing it's done is make my bank account smaller. Plus, and I could be wrong on this, it seems just a little AFC to me. I'll only buy if I want to buy.

Personally, I'd do one of two things:

Tell her you don't see anything you want to buy, but she's pretty cool to talk to and would she like to ... (whatever your thing is - drinks, coffee, whatever).

or

If you think she's pretty into you, leave the store and go to a different one. When you're ready to leave the mall, go back in and tell her you have to go, but would she like to go to drinks, coffee, etc.

This second one I've only tried one time, but it worked for me. I'm also a little different than you in that I tell them that I'm shopping for myself - which obviously wouldn't work at a place like Victoria's Secret...unless that's the type of kink you're looking for. I don't know if saying that I'm shopping for myself hurts or helps, but I haven't run into any roadblocks that I know of.