I have the same problem and after more then 1 month of struggle I found a solution that seems to work for me:
never keep 3g and wifi at the same time on! 3g OR wifi but not both. like this it gets warm but more normal usage warm not hot.
For this purpose sbsettings is very helpfull if it is jb.
Also if your phone is jb then turn off or uninstall auto 3g if you have it!
Also if you have activated push for e-mail let it on fetch because you will see no difference in e-mail app but a whole difference in battery life.
Cloud is also very good battery killer. Deactivate all cloud services that you don't need and if possible all.
It seems that iphone 5 keeps trying to connect to wifi if on 3g and that is killing battery and consuming a lot of resources that gets it very hot.
Now I finally get close to one day with a full charge instead of 2-3 charges/day - at heavy usage!
For car usage I purchased an airvent holder so that the AC blows directly on iphone during summer.
Keep me informed if it worked for you!
Anyway I am dissapointed that it is so much work just to be able to use the phone with most of the features that it was designed with. Like having a super sport car that is not able to run on a race track without damage or problems after each lap.