Settings > Wireless and networks > Mobile networks > Make sure enable always-on mobile data is unticked.
This should improve battery life dramatically. All it does is switch the internet off if you aren't using it (after 20 minutes of no activity). But when you click on the internet icon it should connect to the internet straight away again.
Been doing this with my HTC Desire HD and where before my battery would be at 30-40% after leaving work (5.30pm), its now around 70-80% depending on how much I've been using it.
Originally Posted by double maxim
or just turn wifi, 3g, gps, screen brightness and bluetooth off when not in use like most people don't.
My brightness is set around 36% and its bright enough. Fair enough, you won't see the screen well in direct sunlight but thats like everything (apart from the Kindle).