It's possible that part of the problem is  the very fact that you are using the Apps Manager to kill all apps as soon as you're done using them (if I'm understanding you correctly). Turning off apps manually can actually reduce your battery life. This sounds very counterintuitive to longtime PC users (it has taken me a few months to get used to it), but it seems that that is how Android devices work. The operating system expects to do the app management and memory management, and the more the user intervenes in it, the more energy the operating system uses starting its job all over again. Perhaps this is what dhaynie266 meant in saying that the phone's Application Management Widget, part of the operating system, kills apps automatically.

Moreover, the Android OS (unlike Windows) is constructed so as to keep RAM loaded with apps that it thinks might be needed. Every time we kill an app, the OS will load another one to occupy the RAM, using up battery power in the process. As I say, I've had a hard time getting used to this myself, but it seems that the more we leave it alone, the better the system will work.

There's a good discussion in this thread (with a link to this very informative article).