According to reviews on CNET and All Things D, the app JuiceDefender does work to increase the battery life of a cellphone or media device. JuiceDefender reduces the amount of battery power used by a cellphone by turning off certain functions and managing applications when the phone is idle.
When a phone's screen is turned off, for example, JuiceDefender disables certain types of radio functions, such as Wi-Fi and 3G. These features are automatically reactivated when the phone becomes active again. JuiceDefender also controls how much CPU idle applications use. The app is available in both free and paid versions, with the latter providing more options for power saving.