Right click on the battery icon and go to Power Options. Under "Select a Power Plan" you'll see which plan you have selected (e.g. Recommended, Power Saver, High Performance). Click "Change Plan Settings" and then "Change Advanced Power Settings" under the battery column to get to a menu that lets you change everything that affects power. Turn WiFi to power saving mode if you have a strong signal. Turn USB to selective suspend if you don't have anything plugged in. PCI Express to maximum power savings. I leave the processor at 100%, but you can change this to 75% or lower if you're just browsing. Just keep an eye on the number of programs running. Search and Indexing shoule be set to power saving (windows can organize your MP3 collection for faster searches later). I set my display to the minimum when on battery, and turn it up manually if it's too dim.
Once you have all of these settings tweaked, the computer will automatically use them whenever it's on batteries alone, and revert when you're plugged in.