That’s a great question. A good-quality down bag, such as a Feathered Friends Swallow ($399) isn’t really a purchase. It’s an investment that will last for years—even decades, with proper care. I have a Swallow bag that I’ve had for more than 20 years and it is as good as the day I bought it.
Two things are key. The first is gentle treatment. Of course this is a piece of outdoor equipment, and it's expected to keep up with you. So it’ll survive repeated squishings in tight compression stuff sacks, plenty of nights on the hard ground, and other abuse.
But don’t leave it tightly stuffed when you are home, as in time that will cause the down plumules to lose their spring. When you’re home, put the bag in a large cotton sack so that it can get some air. Try to store in a cool, dry place—especially in moist climates. And when you’re in the field, always try to keep a piece of fabric or other material (such as a sheet of polyethylene plastic) between the sleeping bag and the ground. That will reduce wear on the shell material.
Most of all, try to keep it clean. Grit and sand will jam zippers and grind at the fabric. Dried sweat and bug repellent, meanwhile, will eat away at the lining. Again, keeping something on the ground beneath the bag will help keep it clean. And try to always wear something to bed—a light T-shirt, light long johns, that sort of thing. Not bare skin, if possible. (Sorry, sleepers-in-the-buff.)
If you aren’t counting every ounce, get a sleeping bag liner. In cold weather it will add a few degrees to the bag’s comfort rating, and in warm weather will give you a super-light sleeping option. I like Design Salt’s silk 4.5-oz. Cocoon Bag Liner ($60), which I have used for several years. It’s easily washable and keeps your bag wonderfully clean. Plus it feels great.
Occasionally a bag will need to be cleaned. You don’t want to do this too often, though. Excessive washing will remove the water-repellent finish on the outer fabric. It also will remove the natural oils in the down, reducing their effectiveness.
So use a front-loading washing machine (the agitator of a top-loader can tear the down baffles), and a down-specific detergent. Nikwax Down Wash ($9.75 for 10 oz.) is great as it leaves no residue on the fabric or down. Dawn detergent also is fine—it’s what many bird rescuers use on waterfowl hit by an oil spill.
After washing, dry in a clothes dryer at room temperature, which may take awhile. I’ve on occasion suggested throwing in a tennis ball or two to break up damp clumps of down, but these might also damage the baffles. So let the bag dry on its own.
And there you go! A clean bag. Wonderful.