According to MSL, pillows should be washed at least twice a year (the covers once a month, along with your mattress cover). Check the care label to confirm that your pillow (down or synthetic) can be machine washed - most can be. Use mild liquid detergent rather than a powder to avoid residue. Wash a pair of pillows together to keep the machine balanced and run them through the rinse cycle twice to rinse them fully.
For down and feathers, use the air cycle and dry very fully. Make sure they are completely dry as dampness left in the pillow can lead to mold. For synthetics, use the low heat setting as high heat can cause clumping. Throwing in a couple of tennis balls wrapped in white socks during the dryer cycle helps to refluff the pillows without transferring dyes.
Martha's tips are from the January issue of Martha Stewart Living, part of a six page primer on Bedding Care 101 (starting on page 116) with all sorts of tips and how to's on cleaning and maintaining your bed healthfully.
Happy washing and enjoy those clean pillows!
(Image: Max Kim-Bee, Martha Stewart Living)