Thermos is a popular brand of a well-known device sometimes called a vacuum flask, or a Dewar Flask after its inventor, Sir James Dewar. A Thermos is actually two bottles, one inside the other, with a vacuum between the two. The vacuum helps maintain the temperature of whatever food or drink is placed inside the Thermos, and it is difficult to improve upon a vacuum. By further insulating the Thermos with a secondary means of temperature regulation, you can help the Thermos keep its contents hot or cold longer.
Check to make sure the lid of the Thermos is tightly secured. A tight lid will ensure that there is as little heat transfer as possible.
Wrap the Thermos in aluminium foil, which will help reflect heat away from the outer shell of the Thermos. This will help whatever is inside the Thermos stay cold. Be sure to wrap the lid too, because there is no vacuum there, so that is the Thermos's weak spot. If the Thermos is silver in colour, which is an ideal colour to maximise heat reflection, then foil is not necessary.
Paint the Thermos with reflective paint, preferably silver coloured, to make a more permanent heat-reflective surface. This will help whatever is inside the Thermos stay cold. Some thermoses come with silver exteriors, so there is no need to paint them.
Insulate the Thermos to help keep whatever is inside warm. Your insulating material can take many forms. Extra clothing, such as a sweater, makes a good insulator. Be sure to wrap the top of the Thermos where the lid is. There is no vacuum in the lid, so the lid is the Thermos's weak spot. The material used to insulate water pipes would also make good insulation for a Thermos.