When it comes down to it, electronics are only as useful as the batteries that power them — and today's batteries leave much to be desired. Have hope, though: biobatteries that charge using sweat produced during workouts, as well as ones that gain charge from ambient sounds, could be on the horizon, so maybe you can ditch those Duracells.
If you needed a little extra motivation to get to the gym, biobattery technology in the form of temporary tattoos will be able to transform your blood, sweat, and tears into cell phone juice. Nanoengineers from the University of California recently presented their research on biobatteries at an American Chemical Society meeting, showing that an electrical current can be harvested from one component of human sweat ("lactate"). Devices designed to store energy in a biobattery via this method would also give you feedback about your workout — although, surprisingly, out-of-shape test subjects generated more lactate (and therefore more battery power) than their in-shape counterparts (probably because they were working harder at the same exercises).
Looking to charge your phone a little more... passively? Scientists from Queen Mary University in London, working with scientists at Nokia, have developed a nanogenerator that can turn everyday noises into electrical energy. Tiny, ultra-sensitive nanorods in the nanogenerator are squished around by sound waves (from the noises of speaking, cars, music, just about anything). Those motions are converted into electrical energy which can be stored away in a battery — no workout or sweating required!
It may be a while before these chargers are commercially viable, though. Until then, I recommend my juice-boosting solution of choice: the external spare battery. You can buy one of these puppies for under $20, and as long as you remember to charge it too, it'll carry enough energy to top up your phone and other devices several times. Also, having an external battery makes you the new best friend of anyone else with a dying phone. Keep it out of sight if you don't want to share, I've learned this the hard way!