you shouldnt do it the way rachel says as the powder needs to be mixed with HOT water of 70 deg.
what i did was
get two bottles, one would have cooled boiled water in about half the amount needed for a feed. then in the other bottle put the rest of the amount needed of hot water, add ALL the powder to this and mix then add the rest of the water that was pre-measured out in the other bottle, shake well and this should be a drinkable temp.
ie 3oz of cooled boiled in one bottle. then in another bottle 3oz of hot, mix the powder with the HOT water, then add the cooled water to the hot, will make the milk the right temp and that way the powder has mixed with te hot water to kill bugs etc.
you may need to experiemnt re how much hot/cold, its normally half and half or 1/3 hot to 2/3 cold but always mix the powder with hot water.
i did this when out as well, took a bottle with pre boiled water (half the amount) then took a flask of boiling water with me to put into an empty bottle, added the powder and then the pre-measured cooled water to make it up to the correct amount.
both amoutns of water need to be measured out so you have the right amoutn of water for the amount of powder.
i did this at home as well as it was quick and easy but you can make up formula in advance with hot water, cool quickly under a running tap and then refrigerate. to warm i stood in a bowl of hot water, only takes a few mins.