Pick up pants, go into dressing room, walk right back out and ask her if she can help you get outta your drawers.
I dunno man, best I'd say is probably to just try chatting with her a little in the store....if things go well, ask her if she'd like to chat more later over lunch or coffee or something. If things don't go well, you're just another shopper so no big deal.
Then again....I'm about the worst person when it comes to meeting people/asking girls out, so maybe you don't want my advice.
b