My fiance took me to fred meyer jewlers a couple times. He asked for help picking a necklace for his mom, when I said I liked something he would ask why I think that worked well. I didnt know his mom super well at the time, but this was how he got me my promise ring and later down the road, my engagement ring. The trick is to have someone your gal doesnt know super well, so she can ONLY go off of what she likes instead of the person. If shes on pinterest look over her shoulder? Haha. Or maybe she even had a public one.
Better yet, go off of what youd love to see her in. I actually would have never picked my engagement ring for myself, but I'm super in love with it now because my fiance is. No one ever says this but it means a lot if YOU like it too.