I wouldn't bother with underclocking as it won't affect battery life enough to make a significant difference. Instead, I'd go into the BIOS and if available make sure speedstepping is enabled and that in windows control panel power options the profile is set to balanced or power saving, depending on your preference.
Performance sets the CPU profile to max while balanced allows for a range between 5% and 100% CPU power usage. I'd also set the fan profile in the advanced power saving features to passive unless you encounter thermal issues in which case active may need to be left enabled. There may also be fan settings in the BIOS that will allow for some power savings. None of the power saving features should be enabled if you're gaming or performing intensive operations though as the unit could overheat.
The chances are not good you can even do much in the way of over or underclocking as laptops have little adjustability in their BIOS settings in most cases. Plus, that i3 has a locked multiplier and the i3-370M does not use turbo boost so about the only way to overclock it is to use SetFSB if your motherboard supports that utility. For your CPU, ThrottleStop can accurately monitor it but that's about it so undervolting is probably not possible like it used to be on the old Core2 cpus.