Simplify the home screen
If you're running a device with an AMOLED or Super AMOLED screen, consider a darker background. Black pixels use no power on these panels. So if you choose an image with a lot of black, you can save battery. If you want to make the biggest impact, keep an all black image around for those times when you really must save all the juice you can.
The next place to look is your widget loadout. If you're accustomed to running a ton of widgets, it might be time to stop. Running a widget keeps the app in active memory and allows it to update. If you drop your widgets when times get tough, you can see some savings.
Tweak wireless settings
On most Android phones you can access this option easily. In the main system settings, find the Wireless & Networks section, then pick Mobile Networks. There will be a box labeled "Use only 2G networks".
Selecting this option will cycle the radio off and on. When it comes back, only 2G networks will work. This is one of the biggest battery savers on Android. Of course, you will pay for it with very slow data. But you will have it if you need it.
If you want to go a step further and really take the definition of phone seriously, you can keep voice enabled, but turn off data completely. Having APNdroid from the Market around for this eventuality is a good idea. This app will block your APN settings from connecting, which cuts off data. Voice calls can still get through, but you'll waste no power on data.
Wi-Fi settings are also under Wireless & Networks. If you are not connected to a Wi-Fi AP, you should disable it. Android will occasionally sweep your local area to search for known Wi-Fi access points. This uses a small amount of battery every time.
Take the screen down a notch
First thing, turn off automatic screen brightness. It doesn't really work that well, and uses unnecessary battery. You can do this in the Display menu in main settings.
While you're there, there are two other settings to tweak. When you turn off the automatic brightness, you will get a slider for the manual light level. Crank it all the way down. In the same menu, you will also see screen timeout. The default is usually a minute, but you can turn it down to 15 seconds. That way the screen will shut off if you get distracted, saving power.
If things are getting really tough, check out an app called Screen Filter. We featured it in the Roundup a few weeks back and it can be a big battery saver when you really need it. This app pushes the brightness setting far below the phone's minimum. Just launch it and use the slider to pick a filter level. If you're in a dark space, this might not be so bad, but in lighter settings, the phone is going to be harder to use. These settings make the phone a lot less fun to look at, but we're going for a stripped down experience here.
Background data
You will gain a significant battery boost, but you lose the ability for apps to update in the background and data to sync to your device. Basically, you have yourself a much less enjoyable phone. It's the ultimate in barebones Android.
Don't think we're telling you to run your phone with these settings all the time. This is just an emergency guide to keep your phone more or less usable in extreme conditions. Depending on the methods you choose to use, your phone will still be able to make calls, and access data (sort of). In the end, it's more important that your phone is still on and marginally usable, than dead.