It may not seem very tidy, or even very careful, but the best way to pack your down sleeping bag is by stuffing it into its stuff sack or sleeping bag case. This prevents the same areas from becoming creased or compressed each time (which is what happens when rolled up).
Always make sure your sleeping bag is completely dry before packing it away and leave the zipper open.
Please note: When you get home, or even during your journey, take the sleeping bag out of its stuff sack. This is better for the down and the seams. Give your down sleeping bag a good shake before using it – to allow the feathers to fluff up and reach their full insulating potential.