iPhone is cold

Hello,

I have some cold issues with my phone and maybe you have some suggestions. Went skiing yesterday in about -15°C, turned on Endomondo, wanted to take some pictures, but after about 30mins outside my iPhone 4S gave up to cold and died. And it pretty much happens every time I'm out in cold for about 30/50 minutes. It just can't handle winter.

(Just for trolling... my girlfriends Lumia 520, designed in cold Finland, did not give a shit about anything and worked just fine all the time. And it is even budget phone costing 4x less than my cold, dead thing grrrRRrrr.)

So, maybe you have some suggestions to keep my phone going next time I'm outside in this awesome winter? It was in my pocket most of time so that is clearly not warm enough...

Thanks!