best option for festivals is to buy some old old nokias. battery lasts for days on those things and plus no one wants to steal them!
but if you have to take your sparkly new S4 then you could try putting it into airplane mode whenever you're not using it and turn airplane mode off only if you get separated from your group? you probably won't get a great reception in the middle of a field anyway, which is one of the biggest battery drainers when it's constantly searching for a signal.
also, don't forget to disable sync settings and location settings too.