Yeah, I hear that. Over the past 20 years or so I have tried all sorts of things. Most work to some degree. Some work well for one night, etc…
Generally, I use two bags: A 0F winter bag and a 40F summer bag. Both are good down so they are about equivalent, give or take.
1) Make sure your bag is clean. A clean bag will loft a bit higher and perform better. Old down is fine, it breaks down but very, very slowly. It takes longer to break down than the shell, generally speaking. Water does almost all the washing a bag needs; a small, like 1/2-1/3 as much down soap as is recommended twice per year is good.
2) Hang it, don't keep it in a sack. Let the feathers breath.
3) Wear a down jacket/vest and long johns. This will add 20F-30F to the bag's rating in conjuction with a down jacket(dual use.)
4) Use a good warm pad. Or two if you have them. A lot of heat is lost below you. Nor do you want melted snow below you, wet snow is cold.
5) Stay dry. As Eric mentioned, if you use a vapour barrier, make sure it is as close as possible to you, not over the bag.
6) LIGHTLY drape your cloths over your bag. More than one layer may not pay, though. The loss of insulating value due to compressing the loft may hurt you as quickly as you add it. Good down is not very supportive.
In conjuction with the other stuff already said here, you can extend a bag about 30F, after that, you need a different bag.
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