We pre-make our bottles so I just put them in a cooling area in the diaper bag with some ice packs. I ask for hot water at coffee shops to warm them up in (all the places I've gone to are more than willing to give me a cup of hot water for this)
omg those tommy tippee bottle warmers rock!!! i love it. when we are out and use it i am naughty and pour any leftover hot water into the teapot and get an extra brew haha!
we also premake bottles then reheat (bad parents!)