Every moment of every day, your battery is dying. It dies a little whenever you cruise Facebook or snap a photo, but also whenever it sucks power from an outlet. Batteries aren't passive containers, but bundles of active, chemical reactions, and the more that chemistry churns, the quicker the cells become inert, and unable to hold as much of a charge.
To slow the pace of degradation, avoid compulsively charging your phone. Every five-minute top-up at work or in the car is eating through one of the battery's finite charge cycles, and shrinking its total capacity. Instead, try to conserve energy (using the other tips listed here), and for the truly battery-conscious, consider buying a small, portable external battery, most of which can charge a nearly depleted phone back to full in less than an hour, while you're using it.