I always carry a small unit that has two USB outputs. Search for 'USB power bank' on any popular internet trading site.
The higher the number of mHa, the more charger you will get out of the unit. I get about three or four chargers for my iPhone which will get you through a weekend plus. I mainly use mine if I'm going out on my motorbike. I tend to listen to music via a bluetooth headset and by carrying this power bank I know that I will not run out of charge on my phone.
All you then do is re charge your 'power bank' (via a mini USB input) whenever you are near mains.
There are also
Solar Powered units as well. I can't comment as I bought mine last year and didn't see the
solar ones at the time, but I'm sure you can re charge the power bank either using mains or the sun, so the best of both.