There are two philosophies when it come to packing down jackets: stuff it loosely to retain loft or pack it tight and don't worry about it. If you are like me, and view packing as an elaborate game of Tetris where everything has it's rightful spot to maximize every centimeter of space to avoid checked bag fees, then you'll probably want to go with the latter. If you are hiking in the back country and using the jacket frequently, then it's not really worth packing it down; just loosely stuff it along the sides or to fill space to keep other items from shifting. This will keep the down well lofted and ready for wear...or pack it as tight as you want and give it a few good shakes when you pull it out to fluff it up. Any decent jacket should be able to handle the compression easily.
Below is a step by step guide on how I pack my jacket. I frequently travel with only carry-on baggage for most of my trips, so I put a premium on space.
1. Lay the jacket out flat.
2. Place a bend in the shoulders of the jacket and fold the arms along the outside edge of the jacket.
3. Fold both arms over on themselves until they meet in the middle. You have reduced the width of the jacket by half now.
4. Fold the hood down in the gap created between the shoulders.
5. Fold the entire package in half, using the zipper as a center line.
6. Roll the jacket from top to bottom, starting with the hood. Make sure to compress air out of the jacket as you roll.
7. Place the final rolled jacket in the stuff sack or secure with a rubber-band, velcro strap, or whatever tie you want to keep it from expanding in your luggage or pack.