Go ride it!
Ride lots of bikes. The more the better. One will be the perfect match, and you'll know it when you feel it. You (quite literally) will feel like the bike is an extension of your body, even if you can feel that a couple things need to be adjusted to fine-tune the fit.
Ride lots! Tell the bike shop what you ride now, what you do and don't like about it, and what you want to do in the future. A good LBS is a wonderful thing. Ride anything they suggest, even if you don't think it's right for you. The more bikes you ride, the more you learn what suits you and what doesn't suit you.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson