The best way to dry down is in a tumble drier. Always check the care label of your garment for guidance on heat settings.
Regularly check your garment shaking it out and pulling the clusters apart every 20 – 30 minutes, in between tumble drying. Ensure it is totally dry to avoid any clumps, then air out for a few days.
An easy mistake to make is tumble drying your down garment for half an hour or so until the outer fabric is dry, assuming that the down clusters inside are also dry. When down gets wet the clusters collapse, forming clumps. If left damp garments can be rendered useless next time you go to grab them from the cupboard.
Store it uncompressed and keep it dry. Try to avoid using down in damp environments.