I started backpacking into the mountains over 30 years ago for mid and late elk hunts
. There are some great ideas on here, but the "system" method is the only one that is going to guarantee success. Also listen to the guys who sleep out in the backyard before they go. You wouldn't go without your rifle lined in at home would you?
Here is my 2 cent for a backpack in system I use to this day, bear with me when I repeat the wisdom of others.
1) Get an ultra lite backpacking waterproof, well ventilated bivy. Backpacking websites will list the best of the past and present. You may not use it, but if it rains your bag and you are in heaven. Plus it adds a couple degrees to your system and greatly reduces wind chill.
2) Get a great insulated pad, the warmer the better. It is the foundation your system is built on. Cold bottom, Cold body! Remember air pads cool you down.
a) If you have back issues, there are ultra lite titanium (3#'s) cots that are 5 inches off the ground, out there for Backpackers at about $250. They are designed to fit in a bivy, I always use mine. You can get away with a less heavy pad also, because of the uniformity of the cot.
3) Always have a silk or poly bag liner. No exceptions. Not only will it keep your bag clean and dryer, but it will add 5 to 10 degree to your bag for only 6-8 ounces.
4) IF you have a bag you like, keep it and put it into the system. IF not, any of the great bags mentioned will work.
5) Always dry out any bag to the best of your ability, daily. Even opened and fluffed up in a ventilated waterproof bivy is better than nothing. One thing to remember is to hang it up HiDDEN in a tree if it is above freezing and not raining while hunting. Wind is a hell of a dryer, even when it is cold.
6) Listen to the fella that said always use a set of poly/silk underwear ONLY for sleeping. Hard core backpackers, where ounces count, would never be caught dead without them. They bring two if they are worried about perspiration and not being able to keep them dry. They also wear separate poly socks and hats for sleeping ONLY. They are extremely important, no exceptions.
7) Finally, this little 4 oz, $10 item has made what could have been hell, heaven, several times. An aluminum survival blanket or bag liner. I use the blanket because it is more versatile, inside next to my poly sleeping ONLY underwear if necessary. It will trap moisture, but it will keep you and your bags dry. If you are using it at this point, god has thrown you a curve ball and you will care less if you have to dry your poly underwear. Of course the blanket dries with a flip in the air since it absorbs nothing. I have used it over the top of bags also to keep several people warm in a crisis. DON'T LEAVE HOME WITHOUT ONE!!
The system will keep you warm from 40 degrees to 20 below just by mixing and matching the components and staying dry. A told my system without the poly sleeping only clothing weighs in at 14# -7 ounces and I can go anywhere sleep anywhere. If you are younger and a lot nimbler than me you could get a couple #'s lighter yet. Good Luck to you.