X2 on everything said about ATK. Do a search on it in this forum, you'll learn that it is at best no more help than the stock task manager, at worst, it makes the problem worse.
There are some really good articles out there that explain why it doesn't help.
I can't post links in here, if you are interested in reading them, google "why is advanced task killer bad for android?" and read some of the article hits. There is a FAQ that is very informative, and several other places have good explanations as well.
The power settings widget can help, and there are some good apps in the market as well. I use an app called Silencer Pro. It allows you to set up several different profiles that automatically turn services on or off depending on your needs.
If you're sitting at work, you don't really need the GPS running. That drains a lot of battery. You also may or may not need the WiFi or Bluetooth on, so you can set your work profile to turn those off as well.
You can also manually control these functions yourself with the slidedown status bar, but if you would like to "set it and forget it" this will allow you that freedom, too.
In general, your display will drain the most battery, and the WiFi, Bluetooth, and GPS functions are right behind that.
Once I started managing these services more, rather than just leaving them on, the battery life dramatically improved.