General sleeping bag tips:

Buy a sleeping bag rated for the temperatures you most often expect, not the coldest you expect.

Your choice of ground pad is a major contributor to both your warmth and your comfort while sleeping, and is no less important than your choice of bag. Budget accordingly.

In colder weather, polypro long johns, a beanie, warm socks and maybe a jacket can dramatically lower the useful temperature of your sleeping bag.

Wearing clean clothing to sleep in also helps keep your bag cleaner, and reduces the need for washing, potentially prolonging your bag's lifespan. A Coolmax t-shirt, undies and liner socks weigh only a few ounces.

Down usually lasts longer than synthetics, often much longer if well cared for. When waterlogged, synthetics are warmer than down, although very far from warm. Plan how you intend to keep your bag dry.

Never try to lift up or wring out a bag soaked from either rain or washing; the additional water weight, unsupported, can destroy the internal structures of the bag. Lay it out flat or over something that will support it, like a tent, to dry it.

Thoroughly air out a sleeping bag after every trip, and store it in a roomy breathable bag, NEVER in a stuff sack. Try to air it out every morning during your hike, and remove it as soon as possible from its stuff sack when you arrive in camp each night, which allows it time to reach its full loft and warmth.

Pay careful attention to manufacturer or expert instructions for washing and drying your bag. A mistake here can ruin your bag. Do it yourself to get it done right, and never use a dry cleaner or dryer.

I've never found rectangular bags to be any more comfortable than mummy-shaped bags. Both designs tend to have about the same chest and arm diameter, which is the key dimension for roominess, along with length, at least for me. Mummy bags are lighter for any given warmth rating.

Understand that it is the air trapped by the insulation, not the insulation itself, that provides warmth. Conversely, any dead or leaking air space inside the bag will be warmed by taking away your body heat. That's why snugger is always warmer, at least until it becomes restricting.

A good sleeping bag is an expensive but worthwhile investment. Choose carefully, and be suspicious of design gimmicks and claims of new wonder-fibers for insulation.