Taking your significant other to the store is always a good start but why not have her fill out a simple website? I had my future bride fill out this form:
http://ourweddingproposal.com/index.php?topic=46.0
After she filled it out I knew what kind of budget I might be looking for and what type of ring. Now I plan to get the proposal ready and find a very inexpensive ring. After I ask her I plan to take her to the store of our choosing with a budget and get the perfect ring. I'm not sure how she'll take getting a fake or tiny ring during the actual proposal but I think she'd rather have her perfect ring than one that was "in her mind" her second pick!
Things are getting close just a few more months before I have to have everything in place for my proposal!!
Wish me luck..
Dan
http://ourweddingproposal.com