[ROOT] Meet the CPU Control
Underclocking your device is definitely the best way how to keep your battery for a long time. The battery drain is mostly caused by display and processor. So if you overclock your device, you will save more than 10% of your battery life!
How to do that? Get the CPU Master, underclock your device and you're done!
Please note: If you underclock your device too much, your system will freeze untill you pull out the battery and restart your device. Note also your device can be laggy a bit and sometimes it may be really slow.
[ROOT] Voltage Control does it all
Next, but maybe dangerous way is undervolting your device. If you're about to do that, be sure you have custom kernel that supports this feature. To undervolt your device, get Voltage Control, navigate to voltages section, and reduce voltage of all frequencies down about 45mV.
Note: Do NOT Check the "set on boot" value! Your device may freeze if you undervolt too much, pull out the battery and restart the device.
Screen Saver - Simple but effective
As you know now, the battery drain is caused by CPU and display. To save your battery, lets just reduce the brightness! To reduce the brightness lower than the system allows, get THIS app. This app reduces brightness not only on your display but also on hardware keys!
Battery Saver apps - classic.
Battery savers doesn't really do anything special - they just optimize services like Wifi, Bluetooth, reduce brightness, and so on.