Here is a free one: heat water and fill your nalgene (or other poly bottle) with it and toss into the bottom of your bag as you head to bed. Careful with this as boiling water with no buffer will burn you. However wrapped in a towel etc. it will keep you warm for much longer than you would think.
Also, cram tomorow's clothes in your bag with you- extra insulation and you get to put on warm clothes in the morning. :ylsmoke:
Both are good points. Filling the void in your bag will reduce the amount of space your body has to heat... and things that warm you up are a bonus. I have even gone so far as to stick my feet inside my backpack... inside my sleeping bag (Cold night).
Eat before you go to sleep. Makes a big difference.
Something I have used on the ground while snow camping that may or may not fit your application is pine branches. They insulate well against the cold. When I backpack in snowy conditions, I lay out a barrier of line branches, cover them with a tarp, and pitch the tent on top. I then pack snow around the tent up to a foot or so to provide some insulation and deflect the angle of wind as it passes the tent.
Cooking in the vestibule is always the safer way to do it. No suffocation worries, and your tent does not smell like food when you use it in the spring. I have vestibule cooked several times in rough / winter weather. IMO, cooking inside the tent is asking for trouble for a number of reasons.
If you are camping with your wife, zip your bags together. Sharing body heat in a sleeping bag works well. The_Mrs. and I either use our double bag on car camping trips or we carry our bags separately, then combine them at night.
As stated above. Most important. Stay dry, no cotton. Safety issue.
Sorry to ramble, but many of these techniques work best when used in conjunction with each other.
EDIT: Ventilate your breathing area. The moisture from your breath can compromise the insulative properties of your sleeping bag.