Remember, a nice warm meal/snack before sleeping will keep you warm. Also, before bed, boil up some water, put it in Nalgene bottle, and throw it in your sleeping bag to warm it up. You can also keep it in there with you while you sleep for warmth.
As for down jacket, lots of good articles here about them. You want high fill power (amount of space an ounce of down fills) and high fill weight to total weight ratio. Some manufacturers list fill amount, but most don't so it takes some snooping around to find out. Montbell Alpine Light is a really good one that many people here love. It's really puffy and warm. It was too warm for me so I sold mine and got the Stoic Hadron Anorak instead (half the down, but higher fill power).
Also, if possible, shift the down that would normally be squished under you up to your chest.