A properly cared for comforter can provide years of comforting warmth. The bunching inherent in comforters stems from it's layered construction. Fillers or batting is in place between layers of fabric, and when the filler shifts or the batting deteriorates within the sewn baffles, the comforter appears to be lumpy or brunched. You can use goose down, synthetic fibers, cotton and wool to fill and add loft to a comforter. A good filling material and maintenance plan will keep a comforter from bunching.
Read the manufacturer's care label as soon as you get a new comforter or before you purchase one. Down, synthetics, cotton and wool all have different care instructions. Just because the outside fabric is cotton doesn't mean the filling is.
Restore the air to the baffles or pockets sewn into your comforter by shaking it vigorously or fluffing it in a high capacity dryer with three clean dry tennis balls for 30 minutes. Check the label to make sure you can place your comforter in a dryer before choosing the dryer method. Doing this between washings will extend the life of your comforter.
Follow the manufacturer's care instructions regarding washing. If it is machine washable use the correct fill level, temperature for washing, rinse cycles and wash time. Deviating from the instructions could damage the filler and cause permanent damage to your comforter.
Dry your comforter at the manufacturer's recommended settings with three clean dry tennis balls. The added agitation from the tennis balls will stop the fill from bunching with the baffles as it drys.