I can't really recommend solar chargers at all. They are not great in the UK. I have used a Solio one at festivals, but in all honesty charging it via the sun is most likely to take days. You generally have to keep them aimed at the sun, and you will have better things to do! Most of them do have built in batteries that you can charge directly though.
Better bet is a charger with either a large built in battery, or one that uses chargeable AAs.
The usual power saving tips apply - turn off WiFi, 3G, GPS, etc and even the whole phone when you are not using it.
I would also consider a cheapo handset that sips battery power. I have a couple kicking around that will last nigh on a week without charging. The upside is f you drop it in the mud or get it nicked you won't care.