I'm kinda lazy. I just bring Jerven Thermo Kingsize bag and 3season sleeping bag all year around. String the jerven up with some para, stuff everyting in it, stuff yourself in it and your sleeping like a baby within minutes :p
Temp dropped down to under -25c this weekend and we where the oppisite of freezing. We even made a bed of foliage for more space for activities
Not saying this is the best approach, but it workes for me
I've heard a lot of good things about the Jerven bags. They're not cheap though!