How old is your battery? It sounds like your battery might be getting old.
And then there's my good old tried and true method of getting the absolute most out of my cell phone batteries. Have/buy a second (OEM) battery (or more) and a battery only charger. Do not charge your battery in the phone. I have been doing this for years and years and it has a plethora of advantages:
You never have to plug your phone into the wall. You just let it run all the way down since you always have an extra battery fully charged. You won't wear out the outlet on your phone, after all it is really there for much better uses like hacking and using your phone's sdcard like a USB stick. And last but not at all least, I get much better overall life per charge and longer overall life out of my batteries. I average about 24hrs on each of my regular batteries with moderate use, and the original battery that came with my A855 is in about as good of shape as day 1. And for me Day 1 was release day, November 9, 2009, almost two years ago.
good luck