If you have Usage value increasing even though you are not using it, i.e it's in stanby, like this:
This most likely indicates that a process is running in the background and prevent your phone to really put itself in standby mode.
I had that with my iPhone 4 in the past and I found that it was caused by a VoIP app called Fring that was keeing my phone active, and in fact using a monitoring app like activity monitor touch, I found that not only Fring was keeping my phone up but also eating half of my CPU power all the time.
Normally, Usage should reflect the amount of time you are using your phone, so when the screen is on or when you are calling. It can be a bit higher, because of multitasking background processes, but if you see the "Usage" value increasing very closely to the "Stand by" value, even when the phone was in standby for a long time, then it's most probably a bogus app process running in the background.
Apps that can run in the background are apps using either of the following services: VoIP, GPS (background location), Audio (background audio playing), background downloading.
So try to find the app(s) keeping your phone alive, and uninstall them (don't just kill them, this doesn't kill the background process, particularly for VoIP apps), and see if it's better. Just killing the bad app or restarting the phone or both don't kill the bad process, particularly for VoIP apps that have to be able to receive calls all the time.
