Here is a link to Paul Stewart's advise:
http://badminton-coach.co.uk/4520/what-do-you-think-about-when-choosing-a-new-badminton-racquet/
I have to agree with everything Paul has said, once you have a frame that you are happy with then the only things you need to look at are your technique (go see a coach) and try different strings and tensions!(see me!)