Android (rooted): If your battery won't make it through the day, Greenify is a handy utility that will automatically hibernate battery-hogging apps that linger in the background. We tried it out, and it works like a charm.
I found that it really does help save my Android phone's battery from dying over the course of a day. Once installed, simply add the applications you want Greenify to hibernate when they move to the background, and let the app do its thing. At any time you can see the apps that have been hibernated, the apps that are running in the background, and any pending apps that Greenify will manage when you're finished using them.
It's important to note that Greenify is not a task killer, and it doesn't "freeze" apps. The apps that Greenify hibernates are still usable if you want to switch to them, and you can still pass data to them as if they were still running. When you switch back to them, it's like you never left. Greenify just keeps those apps from launching new background processes (a problem with task killers) and you don't have to unfreeze or thaw an app to use it when you want to (a problem with "frozen apps"). The developer, XDA member oasisfeng, describes how his app differs from task killers and utilities like Titanium Backup over at the XDA forums.
After trying out the app for a while and adding some of my common apps to the hibernation list, I definitely noticed an overall improvement in battery life. It did take maybe a beat or two longer than usual to bring those apps back out of hibernation, but that could have just been my device. We should note that hibernating apps that leverage push notifications and messages or alarms will make those apps not work. Plus, the free version of the app cannot hibernate system applications -- the $3 "donation" version however, can. Hit the link below to read more about it and give it a try.