First off, do not use a task killer to try and extend battery life. You will actually consume more juice using a task killer.
This is a pretty good article to explain why.
First thing you'll need to do is Cycle the battery once. Modern Li ion batteries do not have a memory, but you do have to teach android what a full charge is, so it can report power properly. Let your battery run down until the phone shuts off. Then using the AC adapter, let the phone continuously charge until full. Then unplug the phone, and shut it off if it's on. Plug it back in. If plugging the phone it turns it on then while it's plugged in, turn it off and let it charge for at least another hour, even if the phone says fully charged. Reboot the phone.
Be advised that you only need to do this once as repeated deep discharge cycles can harm the battery.
For general power saving, turn off WiFi, Bluetooth and GPS antennas when you aren't using them and
set the screen brightness to low or auto. Also, don't sync data frequently. gmail is push, so that won't matter, but any widgets or mail accounts that sync data should do it only once an hour or less.
