There are several strategies for cleaning stainless steel thermoses under special circumstances. The daily cleaning of one, however, is rather straightforward and requires no extra cleaning supplies than those you might use for your dishes. Thermoses should always be cleaned by hand and not put in the dishwasher. According to The Gourmet Kitchen website, "These stainless steel cleaning tips should work on any thermos product, as well as larger stainless steel cookware like pots, pans, and Crock Pots."
Soak your thermos in warm, soapy water for a few minutes if it is particularly soiled. This will loosen any remaining food particles.
Apply a dime-sized amount of gentle dish-washing detergent to the thermos.
Rub all surfaces, inside and out, using a soft cleaning rag.
Rinse with warm water, until the soap is completely diluted.
Dry the thermos using a dry cloth or paper towel.