I take sterilised bottles, however many cartons I will need, a mini thermos flask of boiling water and a small plastic cup. Then at feed time all I have to do is pour the milk into the bottle, pour some of the boiling water into the cup, then pop the bottle in the cup to heat it up. I use Aptamil but if you use SMA you can buy ready made formula in disposable bottles (you buy reusable teats separately) so you don't even need to take bottle with you.