In addition to what Susan and Karin suggested, you should try to attend as many workshops and seminars as you can. There is a lot of new information that comes out continuously, so you must take advantage of it. Usually whatever certification(s) you decide to add to your credentials, they offer many great workshops, seminars and conferences with a lot of information about fitness and nutrition. Most certifications will actually require for you to sign up for a number of such events as part of their continuing education requirements.
Good luck with your search!