In the Store: Choose down when warmth-to-weight and compressibility matter most. But remember: Get feathers wet and your bag is dead weight until you find a dryer. Opt for synthetic fill (or waterproof down) when you expect frequent condensation or dew. » Check fit: If you sleep like a rock, a narrow cut is preferable because it's more efficient (less air to keep warm). If you toss and turn, go for comfort, even if that means more weight and bulk. » Everyone: Get a bag rated 10 degrees warmer than the coldest temperatures you expect.
In the Field: If possible, air your bag out every morning in the sun. » Cold snap? An aftermarket silk or synthetic bag liner can add 10 degrees of warmth. » Line your nylon stuffsack with a garbage bag for added moisture protection.
At Home: Dry your bag in the sun before storing. » Prolong loft by leaving it loose, not stuffed. » Don't be afraid to wash your bag (even down) when needed; just follow manufacturer instructions and dry thoroughly.