The most effective and simple way probably is to setting your gadget into flight mode, if you don't plan on answering the phone in a long period of time. Also, charge your phone before its battery running all the way down. Lithium-based batteries are designed to be charged early and often. There are still other things you could do to prolong battery life: avoid using unnecessary features (using moving or animated pictures for background is not suggested), turn off Bluetooth/WIFI/GPS and avoid leaving your phone in a hot car or direct sunlight.