1. What is the coldest weather you will be camping in?

If your backpacking is limited to the summer months, you can probably get away with any sleeping bag rated down to 40 degrees Fahrenheit (4 degrees Celsius). Otherwise, consider your future plans and get a bag that will work at the coldest temperatures you expect to encounter. If it is too warm for summer use, you can always open it up like a blanket and lay it loosely over yourself so it won't hold in so much heat. Another alternative is to buy one bag for winter trips and one for summer.

2. What kinds of conditions will you be camping in?

The conditions you can expect will determine what fill is best for you. If you will always be backpacking in the northwest, and so possibly getting wet most of the time, you don't want down. It loses most of its insulating ability when wet. A good synthetic fill is better. On the other hand, if you will be camping most of the time in the desert southwest, a down sleeping bag is the lightest (and most compressible) option available. My own down bag weighs just 17 ounces, stuffs into a bread bag, and has kept me warm to below freezing. If you are careful you can keep any bag dry, but it is tough in some conditions, so ask yourself where you'll be going, and how good you are at staying dry.

3. How claustrophobic are you?

Some people really can't get comfortable in a mummy bag. They need room to move around. If you aren't sure how you feel about this, there is one way to find out: get in those bags. See if you feel good in a snug mummy. Borrow one and spend a night in it if you have to. A good mummy is the warmest sleeping bag you can get for the weight, but semi-rectangular may be better for comfort.

4. How big are you?

If you are tall, be sure you can fit comfortably in that sleeping bag with the hood closed. What if you are too big for a tight mummy bag? Try a rectangular or semi-rectangular one.

5. How heavy is the sleeping bag?

Even if you are not into ultralight backpacking, there is no reason to carry too much weight if it isn't necessary. A decent summer sleeping bag shouldn't weigh more than about three pounds. To get down to the weight of my down bag (17 ounces) you'll have to spend more. A winter bag will be heavier because of the additional filling, but in any case, compare the various bags according to their temperature ratings and total weight (with the stuff sack if you'll be using it).

6. How packable is it?

No filling invented yet is as compressible as down. For that reason, if you use a small backpack, you may want to consider a down sleeping bag. Some synthetics can be packed in compression sacks to reduce their volume, but this can eventually damage the filling and so reduce the bags insulating ability. Also, a compression sack adds more weight to your load. It is probably better to simply put a large synthetic-fill bag on the outside of the pack.

There are certainly other specific questions to ask about sleeping bags as you look at them. A zipper that opens at the bottom is nice for those with hot feet. A water-resistant shell can keep dripping tent condensation from wetting the filling of the sleeping bag, or allow dew to be shaken off in the morning if camping in the open. There are many other features you might find too, but start with the six questions above to quickly narrow down the options.