WE FEEL YOUR FRUSTRATION. Every day you're bombarded by a cacophony of so-called fitness gurus, diet proselytizers, and doctors with magic pills, all claiming to have the answer to your performance needs. It's enough to make you throw up your hands and succumb to Starsky & Hutch reruns. So let us help. We surveyed more than 50 of the country's experts in exercise science, coaching, nutrition, equipment, and health to find out the best trends for 2004. What does this list mean for you? A spiced-up conditioning routine, guaranteed results, and a whole new way to think about fitness.