ScreenDimmer is a jailbreak extension that lets you manage various brightness and backlight settings on your iPhone, iPod touch or iPad to help you conserve battery life. All you have to do is configure it the way you want and it'll do its work in the background.ScreenDimmer is an extension so it appears in your iDevice's Settings app. Once you've purchased (for about $US2), downloaded and installed ScreenDimmer through Cydia, you just need to scroll down the main page of the Settings app to find it. Tap it, and you'll be able to toggle a bunch of settings that control how your device's screen operates. You can have the screen dim after a specified number of seconds, or have the backlight turn off entirely. You can keep the screen on when the device is plugged in, lock immediately when the device is unplugged and more.
If you're looking to eke out a little more battery life from your jailbroken iDevice, keeping the screen dim or the backlight off entirely is a good way to do it.