The feathers inside a down comforter accumulate moisture, soap residue and oil over time, causing the feathers to stick together. The feather clumps then shift to one side of the comforter, leaving that side lumpy and the other side flat. The best way to redistribute the feathers is to wash the down comforter properly. Most home washing machines aren't large enough to properly clean a down comforter. Your best option is to take it to a laundromat with industrial-sized machines. It is worth the trip and expense for a cleaning that only needs to be done once annually.
Add detergent to the washing machine. Use a detergent listed for use on delicates. Pour in the manufacturer-recommended amount for a light load. Too much detergent will damage the feathers.
Place the down comforter by itself in the machine.
Turn the machine on to its "delicates" setting, if available, or simply set the water temperature to "warm." After the first rinse cycle is completed, run the comforter through a second rinse cycle. In washing machines with knob controls, you can do this by simply turning the knob back to the "rinse" setting. If you are unsure how to do this, ask the attendant for help. After the second rinse, allow the washing machine to finish its cycle (spin dry if possible).
Place the comforter in a large-capacity dryer and set the heat on low. Add two clean tennis balls to the mix to help fluff the feathers.
Remove the comforter from the dryer every 15 minutes or so, and shake the feathers around to fluff them. If you have a friend with you, each of you can take one end of the comforter, and shake it up and down to get the feathers moving.
Take the comforter out of the dryer, and hold it up so that the overhead light shines through it. If the feathers still look clumped together, they are not completely dry. Repeat steps 4 and 5 until the feathers are fluffy and distributed throughout the comforter.
Down comforters should not be washed more than once annually. To keep your comforter clean in the meantime, cover it with a duvet cover, which can be washed much more frequently. Instead of the second drying cycle (you will need at least one), you can hang the comforter out to dry on the line on a bright, sunny day.