>> Calculate the lowest temperature you expect to encounter. Now subtract 10 degrees. That's your target rating.
>> In the store, lay the bag out and crawl in. Zip it up, snug the hood, fiddle with the drawcords. Is it too long, creating excess weight? Too short, causing compressed insulation and cold feet? Does it have enough wiggle room? Efficient mummy shapes (read: snug) might save weight, but they only make sense if you get a good night's sleep.
>> Close your eyes and get out, as if you're making a midnight exit. Is it easy?
>> Run the zipper a few times–from both inside and out–to make sure it doesn't snag easily.
>> Inspect the loft. Are the baffles puffy and evenly filled?
>> For cold-weather bags, allow more girth for extra layers and space at the feet for hot-water bottles.
>> Do you like to sleep under the stars or a tarp? Be sure to get a bag with a shell that's water-resistant. In the store, ask if you can wet the bag with a spray bottle. Does the water bead up?
>> In the field, when possible, air your bag out to let body moisture dry.
>> At home, hang your bag in the sun to dry it thoroughly, then store it in a large, breathable sack or pillowcase.