Here's some advice I gave someone who asked how top open in Victoria's Secret, but it applies to any clothing or accessory store and with some tweaking you can even use it for stores with other things.
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Whenever I open in a clothing store my patented method is to find a piece of women's clothing and hold it up on myself and ask for her opinion. If the girl I'm interested in is with another girl, then I'll ask the other girl first, then the girl I'm into and if she's with a guy I will do it the other way around. Depending on what is at hand I might pick anything as ugly as a bright flowery moo-moo, or something sexy like pink and black velvet and lace corsets.
The key is to ask her as if you're serious about needing her opinion and considering purchasing it. Even though you're acting as if you're serious though, every now and then I'll crack just the slightest little quirk of my lips and crinkle my eyes a little in a playful manner. This lets them know I'm not utterly insane or creepy, I'm just having fun and playing with them, so they'll play along and we'll go around looking at various things together.
If it's something that is sexy, then I'll say something like, "Hmm...this might look better on you." and then hold it up on her and check her out to see if I like it or not. If so, then I'll say so, which works especially well if she's looking for something like that (this will work perfectly for your situation). I'm not shy about it either, I'll put my hands right on her body when I do this to show how confident and comfortable I am as well as to initiate some kino and make her comfortable with it. Once I've done this once or twice I'll just start helping her pick things out and only once in a while joke about how it would look on me.
Basically the idea is that I'm playing up the gay friend act, even though we both know that I'm not because I'm obviously playing. The fact that I'm playing this character (don't act gay, just assume the role of the gay friend, no stupid gay voice or gestures or anything) I'm allowed to touch her like a gay friend would and she feels completely comfortable.
Like Chief, this is field tested in several different lingerie stores and regular clothing stores as well.
I completely agree with Bonita though, if you're going to approach in a store you should do it right away, because you're just creepy if you're watching her across the store. If I'm actually shopping with someone, or for something, then I may look at her once and go back to my own thing for a while and know she's close by I'll look at her and then approach because walking all the way across the store is weird unless she's the only girl in there (then you have the excuse of wanting her opinion and there wasn't anyone else to ask).