Switch your 3G, bluetooth and "Location Services" OFF
Always hit the top button when you aren't using it.
Turning screen brightness down and switch auto-adjust off.
Probably best to switch wireless off when your not using it, especially if you are in range of a marginal wireless signal (I don't switch this off generally).
Try not to tuck it in a trouser pocket, if it maintains a good mobile signal connection there is a lot less battery drain on renegotiating a cell connection.
Batteries are not good on these things, one day is about the norm and not even that if you dare use it.
I'd approach TheDr at Bigabyte if I really wanted someone to fit a battery.
40 hours is only achievable if it maintains a good signal and stays in the same cell and has no usage and at 25 degrees centigrade etc etc etc.
bluetooth and location services are off, i do lock it when not in use anyway, wireless is only on when i'm indoors, i turn it off when i go out, and if i'm not meant to put it in my pocket, where do i keep it??? and by 3G i didn't mean it had 3G capabilities, just that it's the 3rd gen one.
Batteries can be replaced, it's easier that you would imagine, but it'll invalidate any warranty.
i bought it used off ebay, i'm not too arsed about warrenty lol