Turn off all the auto updates... like the weather email and stocks... There are alot of things that update in the background.

Also, if you aren't using wifi, or 3g, I turn them off too.

The other day I had just less than 50% left, then a few hours later of being on stand by It was telling me I had a low battery. Once I changed everything to manual, then I haven't had any issues. With 2 bars left, I was playing online for over an hour on Wifi before the low battery warning came up. I hadn't charged the phone in 2 days at that point.

good luck.