I worked as a VW mechanic for 10 years and hate the cold! Some of my tips for
staying toasty are:

Air cooled:
 - Put extra non flammable insulation. non water absorbing on the main duct
from the heater boxes.
-  Add non flammable, non water absorbing, heat proof insulator on outside of
heater boxes. I used to use this muffler wrap that hardened with heat, but
haven't seen it lately.
- Fill all the hollow spots behind the panels with fiberglas or foam make
sure it can't absorb water or it will cause rust. On the front panel under
the dashboard
I used styrofoam packing pellets. I could just feed them in by pulling the
panel out a little.
-Put 1 inch carpet foam under the front and rear mats.
-When it got around 20 degrees or less I used to put a clear plastic sheet
behind the front seat to keep the heat up front and block the heater vent for
the rear.
-Make sure the slider vents at the  bottom back edge  on the front doors are
open
-Various seat treatments I've used are.  Reflector sheets such as space blanke
t material, covered with sheepskin or regular seat covers. I currently use a
heated seat cushion with massager which is fantastic, they have them at Sears
for $40.

Waterboxers
-Put closed cell pipe insulation around all visible heater hoses on the
underside.
-When the temperature averages less than 40 degrees I cover 3/4 of my drivers
side air intake with a piece of sheet aluminum that clips over the plastic
louvers.
Remember to remove in warm weather or you will over heat.
- Fill all the hollow spots behind the panels with fiberglas or foam make
sure it can't absorb water or it will rust.
--Put 1 inch carpet foam under the front and rear mats.
- Reflector sheets such as space blanket material, covered with sheepskin or
regular seat covers. I currently use a heated seat cushion with massager
which is fantastic, they have them at Sears for $40 and worth 3 times the
price, what I originally paid!