SetCPU is your under-clocking tool. You can either buy it for two bucks on the android market or download it from http://forum.xda-developers.com/showthre.... Please buy it from the market to support the developers as a donation.

SetCPU requires root privileges, so go ahead and use unrevoked's root tool and start up SetCPU.

You will need to set up some settings. First of all, set up and enable profiles (you will disable this when you need more power for some games). Profiles is the second tab in the SetCPU tool.

Add a profile. The condition is "Screen Off". Set the MAX speed to the smallest value using the slider. Same with the min speed. Set the priority to 100 using a slider. This means that when the device has no screen display, it will be in absolute minimum power mode and still on. It can't increase power.

The priority controls which profiles take precedence, higher priority with a met condition means it will be used vs a lower priority. So keep this in mind.

Next I set the condition to Charging AC/Full and set the speed to 100% max, lowest min, priority 99. This ensures that when my car charger is plugged in and I am using GPS navigation, I get max juice. Note: the screen being off will take priority here.

Next is a 70 priority when the battery is <10%. Set the same configuration as screen off. This will ensure that when the battery is running low and you conserve battery life so you can at least make a call or look up a number.

Now, set your slider on the first tab (Main) to say 499 max. This will be your default mode of operations. Most of the time, 50% power is more than enough. If you notice no slowdowns when doing normal work, feel free to lower that number. When playing a game that slows it down, just set that slider to something higher.