Perhaps the purists won't like this but to keep warm in the bush (and around town) I have used petrol powered (lighter fluid - don't use real petrol from the bowser) hand warmers like the Peacock model. http://www.peacockhandwarmers.co.uk/
I have even kept one in my sleeping bag in the snow.
I see Zippo now have a model which looks pretty interesting.
Note that there are a lot of el-cheapo imitations out there. If you are going to have petrol burning in close contact to your body I would not personally trust a cheap imitation. Also note that there is no flame involved with the genuine ones - the heating comes from a platinum catalysed reaction but the reactor bed does glow red-hot.
Has anyone else used these?