first off, when you open a bunch of programs, they can run in the background and need to be closed for the best battery life, using hte "X" in the upper corner. Doing hte hard reset like everyone suggests will do this, it's just like your home computer, you wouldn't open a bunch of programs at once and not close any of them right?
Install these two cabs in this thread, tehy will reduce the memory usage, reducing how much the phone is doing stuff in the background. Read what "no push internet" does before you install it.
http://forum.ppcgeeks.com/showthread...ight=cabs+save