It might be a stretch to call this "going green", it isn't on par with using reusable bags or replacing your light bulbs with energy efficient bulbs, but these are a few tips tohelp you maximize your batter charge!
Turn off unused connections when not in use for example: Wi-Fi ® Bluetooth® (Manage Connections)
Use “Auto On/Off” (Options>AutoOn/Off)
Use a single mode only. Choose 2G or 3G rather than 2G and 3G. (Options>Mobile Network)
When exiting, be sure to close the app, not leave it running in the background. (close with the ESC key, the rounded arrow not red power end key. Or, choose Menu button and scroll down to close)
Log out of applications when they are not needed.
Reduce Backlight Timeout (Options> Screen/Keyboard)
Reduce Backlight Brightness (Options > Screen/Keyboard)
Use a device holster (this turns off LCD automatically when you put your device away)
Set Audible Roll for your trackball to mute (Options> Screen/Keyboard)
Key Tone Off (Options> Screen/Keyboard)
Turn off flash (Camera > Options)
Set picture size to Small (Camera > Options)
Set Color Effect to Normal (Camera > Options)
Set Picture Quality to Normal (Camera > Options)
Turn off GPS when not required (Options > Advanced Options > GPS)
Use the zoom out function to get the largest viewable area that you can still use usefully.
Close browser when finished using (close with ESC key, not red End key)
This is pretty big list, do you have any additions? Or maybe just some general "green" tips for life?