If you do something stupid at -40 , you die.
It is very easy to do somethging stupid at -40.

Never done that myself (the -40 bit, I am proficient in stupid)
but have met several that have and discussed the topic many times.

Some bits :

Avoid metal bits, they will burn the skin.
Avoid fiddly bits that need bare hands to be used

It is very difficult to keep a down bag from accumulating perspiration and then freezing.
At some point sublimation comes into play. I know that at the South Pole at least it has been used to dry sleeping bags and clothing, if I remember correctly you need temps below -40 and wind.
(however a down loving mate has used only TNF synthetic bags in Alaska and Minnesota and outside the station at the South Pole, the other guys there did the same. Worked well inside a full eVent bivy on several trips)

Always a good idea to see what the locals use, see Heather's suggestion..(add Wintergreen to that)
Wool and fleece over down.

Have spare hat and gloves.
Make sure that the gloves (mittens over gloves) are easy to put on and take off and must have workable idiot cords.
(not the cosmetic type...)

Just to add to the fun...
At -40 a 5 mph wind (yes 5...) will cause frostbites in 10 min.
If you are not particularly after a nose job you need to cover that and every bit of exposed skin up.
( a 15mph wind will do that in 5 minutes, take your time...)