There are many methods available for cleaning flasks. Some models work well with a bottle brush or there are dishwasher-safe flask options, too. However, first and foremost, look to the manufacturer's recommendations for cleaning and care. Going against these recommendations could void a warranty or lifetime guarantee, and is likely the best way to care for your particular flask. A copper flask and titanium flask will clean differently than food-grade stainless steel, for example. Methods that you'll find online include vinegar, baking soda, soap and warm water, and more. Again, we recommend following the manufacturer's recommendations for cleaning.