In most cases, the most important thing is to make sure that you clean your thermos bottle on a regular basis, specifically after each use. Most manufacturers recommend hand washing, some of them to allow for use of the top-rack of a dishwasher for some parts.
In most cases, a simple rinse with water after each use and a wipe with a damp sponge can be all you need out in the field. Once you get home, take the thermos apart completely and wash in hot, soapy water. Make sure to rinse the soap away completely, and dry the thermos thoroughly. Doing so ensures that the seals are maintained, and helps to prevent leaks from occurring.
It’s also a good idea to regularly inspect the gaskets, seals, and smaller parts for wear and tear and replace them when needed. Finally, make sure that the thermos bottle isn’t put through too much abuse.