Samsung's two power saving modes are your best bet for squeezing extra hours out of your remaining juice.
Regular Power Saving mode, enabled by going to Settings > Power Saving > Power Saving, will lower the processor clock speed and dim the screen even further than the lowest brightness setting. You can also set it to restrict background data, preventing apps from using 3G or 4G when the phone is in your pocket, and enable greyscale mode to make the AMOLED screen even more power efficient. You should get severalextra hours of use, although it won't be quite as nippy because the CPU is running at a reduced speed.
For those long weekends, festivals and trips abroad where you won't have access to a power socket, you'll want to use Ultra Power Saving to get you through. Enabled by going to Settings > Power Saving > Ultra Power Saving, this automatically switches the phone into greyscale mode and limits which apps you can open. You can still phone and text, and even use the internet, Facebook and Twitter, but that's pretty much it; you can't take pictures, can't listen to music and can't watch video. With less than 10% battery remaining, we went an entire extra day before needing to find a plug socket.