Damp is the biggest enemy to down & feather products. Try to keep your down products cool & dry all the time. A dry environment will keep the down from getting clumpy, and extend the service life of down & feather products.
Grease (from the human body) is also harmful to down products. Therefore, duvet cover, pillowcase, and bed sheet are recommended to be applied on all down beddings to keep the product from the greasy stains.