Turn off your wifi/put it on airplane mode. Turn your brightness all the way down. Turn all your sounds off. Try not to play any games because then you will just keep getting the '20% battery left' alert. Keep it locked and only use it when you need to and use it fast. Then run inside your house like a mad person to charge it. When it's charged/charging, you can put your wifi back on, turn up the brightness, and turn on your sounds.
Lower the brightness, Lower the sound, Download a "Battery Manager" app. Any kind. There's plenty in the app store. And I suggest you get the free one. This application allows you to see your percentage you have left and how much you need to make it 100%, etc.