[Desire General] MasDroid:
There are loads of "tweaks" you can make to improve battery life of your handset... you can try the following and it should make a huge difference for you, and result in longer battery life span:

1. Lower screen brightness. Go to Settings -> Sound & display -> Brightness. Slide the brightness switch to the left to lower it or check the "Automatic brightness" box to let the phone use its sensors to adjust the screen according to the ambient light. You can also find dedicated screen brightness widgets in Android Market.

2. Turn off 3G, WiFi, GPS, and Bluetooth when you don't need them. The easiest way to manage these wireless radios is to use the Settings widget preinstalled on your phone. Tap the + button on your home screen -> Widget -> Settings. Once your chosen widgets are on any of your home screens, a simple tap turns each one on/off. You can also use "Power Control" widget found in home screen -> Widget -> Power Control. There are other widgets that do this in Android Market (e.g. SwitchPro), but these are already preinstalled and they're a nice-looking set.

3. Disable WiFi and 3G network notifications. Go to Settings -> Wireless & networks -> Wi-Fi settings. Uncheck the Network notification box so your phone isn't constantly scanning and looking for open networks to tell you about. For 3G, go to Settings -> Wireless & networks -> Mobile Networks.

4. Disable "always on" mobile data. This is one of the biggest battery savers. Go to Settings -> Wireless & networks -> Mobile networks. Tap "Enable always-on mobile data" to uncheck the box. I haven't found a negative consequence of disabling this feature, as Gmail and other apps/functions still work perfectly.

5. Turn off background data. If you're not using Google services, go to Settings -> Accounts & sync and uncheck the Background data box so that applications cannot sync, send, and receive data whenever they want to.

6. Lower the screen timeout interval. The phone's screen can be set to automatically turn off when you don't interact with it for a predetermined amount of time: 15 seconds, 30 seconds, 1 minute, 2 minutes, 10 minutes, or never. Depending on what you're doing, a too-short interval can become annoying (especially if you didn't disable your lock screen), so choose wisely.

7. Manually turn off the screen when not in use. Since the screen timeout function ensures that the phone's screen will turn off (unless you have it set to "never turn off"), it can be tempting to put the phone down when you're done using it and just let the screen turn off by itself. Instead, give the power button at the top of the phone a quick push to turn it off manually.

8. Turn off auto-sync. Go to Settings -> Accounts & sync -> Auto-sync.

9. Change auto-sync frequency (Weather, News, Stocks, Twitter, etc). If you don't want to turn off auto-sync, you can adjust how frequently data is retrieved/updated. The settings vary depending on the app (and some don't offer the setting at all), but it's generally found in the same place: Settings -> Accounts & sync. Select an account from the list and then tap "Account settings" to change the sync frequency.

10. Disable wireless network location services when not needed. Go to Settings -> Location -> Use wireless networks. Note that this (or one of the other options) needs to be enabled if you want to see and/or use your location in apps like Maps.

11. Turn off window animations. To turn it off, go to Settings -> Sound & display -> Animation -> No animations.

12. Don't use a live wallpaper. Use a static wallpaper instead.

13. Use a dark wallpaper. Some say that dark wallpapers are more energy efficient than light wallpapers. Others say it doesn't make any difference. Either way, there's no harm in using a dark wallpaper so you may as well do it.

14. Change WiFi sleep policy. This may sound counter-intuitive, but leaving WiFi on when the phone isn't being used and the screen is off is actually better than letting it sleep. When WiFi sleeps, 3G wakes up to sync, get email, and retrieve other data. 3G will eat up more battery than WiFi, so go to Settings -> Wireless & networks -> Wi-Fi Settings. Press the Menu button and tap Advanced, then Wi-Fi sleep policy. Choose "Never" from the pop-up menu.

15. Monitor your running services (or consider using a task killer). Just because Android 2.1 is supposed to an intelligent operating system that will manage your running apps for you doesn't mean it will always do things the way you want them to be done.

You can take matters into your own hands by going to Settings -> Applications -> Running services, where you can view what your phone is doing and manually stop certain activities. You can also download a third-party task killer from Android Market and kill selected apps to free up some resources and potentially save some power. Task killers are a polarizing topic in the Android community, so tread lightly when looking for recommendations or asking for help with them.

16. Choose your home screen widgets carefully. Widgets that need access to the internet to update information (e.g., Facebook, Twitter, Engadget, Slacker Radio, Weather, Stocks, News, Mail) can suck up a lot of juice if not managed well, so choose them carefully and set their sync/update frequencies to a reasonable interval.

17. Disable notification lights. This may not be practical if you're someone who really needs/wants to know when something happens on your phone when you're not looking, but for everyone else, feel free to turn off those notifications. Go to Settings -> Sound & display -> Notification flash and uncheck all the boxes.

18. Turn off phone vibration. If the ringer is on, why do you need the phone to vibrate? You're already in a loud environment that drowns out the ring; answering the call won't quiet your surroundings so you can hear what the person at the end of the line is saying.

And if your phone is set to silent, then an incoming call should be silent. Yes, the buzzing vibration may not be disruptive as your ringtone, but people still hear it.

So go to Settings -> Sound & display -> Phone vibrate and turn it off.

19. Turn off audible touch tones. In Settings -> Sound & display, uncheck Audible selection and turn off Audible touch tones.

20. Turn off haptic feedback. Once you get the hang of typing on the phone's on-screen keyboard, you can turn off the haptic feedback that was turned on by default. Go to Settings -> Language & keyboard -> Touch Input -> Text input -> Vibrate when typing.

You can disable haptics in other areas of the system by going to Settings -> Sound & display -> Haptic feedback.