Clean it after every use, using a dish brush and washing up liquid. You can remove the seals in the stopper and clean them separately. If the product is difficult to get clean, use dishwasher detergent or bicarbonate/baking soda together with water and let it soak overnight.
Please, do not wash your vacuum flask in the dishwasher as the coating could become discolored. Also as the small hole doesnt let enough water into the flask to clean it properly.