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 Control widget to control the wireless connections, GPS connection, Bluetooth power, display brightness, and syncing activity.