Turn off radio communications that you aren’t using. If you aren’t using Wi-Fi, Bluetooth, or GPS, use the Settings menu to turn them off (Note that the GPS receiver is only turned on when you’re using an application that makes use of it).
Turn down screen brightness and set a shorter screen timeout.
Turn off automatic syncing for Gmail, Calendar, Contacts, and other applications if you don’t need it.
Use the Power saver feature in the Settings menu or the My Settings widget to control data activity, Wi-Fi connection, Bluetooth connection, and power used for the device such as screen brightness and phone ringtone.