If you have a great sleeping bag, you'll enjoy the trail much more. After all, you'll be spending at least 8 hours in it every night. Its no fun shivering. Improve the temperature rating of your bag by 10 degrees with a bag liner. Some are made of silk, or you can make your own by simply folding a 60 inch wide by 76 inch (2 yards) piece of fabric in half
and stitching the foot bed and length with a running stitch. This can be done by hand as well.
I have a 30 degree, 800 power fill down bag that has lasted hundreds of trail nights, and after a good washing in 2008, is ready for another hundred. A sales clerk who'd also thru hiked the Appalachian Trail suggested this solution to my Bag Quest. I wrote more about my Hydrogen Marmot bag at Gear Reviews
The first decision is down or synthetic. Previous to my AT hike, I'd been worried about not being able to keep it dry. After all, they say the difference between down and synthetic, besides weight, cost and pack- ability, is when wet synthetic will keep you warm. I've never had to find that out. I guard against wetness, as though my life depends on it. It does.
So, after coming to that conclusion, I bought my down bag in Damascus while hiking the trail. It weighs 24 ounces, $269 bucks at the time. I've never once regretted that purchase.
Of course a good down bag will cost you more than that now. Check out the sales at REI, become a member and get dividends end of the year. Get a good bag at Campmor, or the Porche of all bags, Western Mountaineering.
Personally, I wouldn't consider sub 750 fill because of the weight and quality. If you take care of it, it will last.
If you decide to go synthetic, buy quality. You should be able to find a synthetic bag for half the price of quality down bags, a consideration if you can handle the weight better than the cost. A thirty degree bag is a good choice, 20 degree is overkill for most hiking, 40 degree is too light in the mountains.
Don't ever use a compression stuff sack for your bag, down or synthetic. Use a silnylon stuff sack which has been lined with a sturdy plastic garbage bag, stuff and carry it inside the pack. Use a pack cover in the rain. The four layer system will keep your bag dry.
Never trust a "hot water" bottle inside your sleeping bag. Contrary to advertisements, even those Nalgenes will burst, and leak. If you're going to cook near the bag, take care no sparks put holes in the fabric.
Always put your tarp or tent (the shelter) where you can grab it in the rain without exposing your sleeping bag. Once your shelter is up, pull out the bag and fluff it. Reduce compression of the loft and you will increase both warmth and lifespan of the bag.
If you get to camp and the sun is out, hang the bag outside to air and fluff. This helps prevent moisture build up and freshens it without washing.
Follow washing directions on the label which comes with your sleeping bag. Even if you sleep in clothing or a bag liner, body oils, dust and pollen can degrade the insulation. A hand washing in the tub with gentle soap (even dish soap) and drying completely will improve the insulation value of an old bag.
Get a bag to fit. The above video is posted on my YouTube channel:
and embedded here for your enjoyment.
Over the course of my trails I've made bag liners from army blankets on trail because I started with a worn out bag. An ounce of prevention is worth a pound of cure. That was my literal experience.