Radiator insulation
 

Any thoughts about a fire blanket draped over the radiator in between drives?
Anyone done something similar? Safety concerns?

That should work... I was driving many cars with a fire blanked lying on the top of the engine to keep it warm during winter, but not only then, and it worked great. So i can;t think on anything that can be a safety hazard. Untill you won't start driving with a still tightly wrapped radiator of course. :)

Numerous members have insulated the engine compartment. A coolant heater is a good idea too, obviously (if you have access to mains power).

The more HEAT you're able to retain/hold in the engine/coolant, means the less time & fuel needed upon restart to get the engine BACK UP to its efficient operating temperature.

heat and mpg
 

In a May 20,1991 AUTOWEEK article,they mentioned SAAB's thermo accumulator testing which would provide 'warmup' all the time.
MPG gains were 8%.
Emissions reduction,12%.
Volkswagen has also played with Schacht's eutectic salt heat storage system for conserving power plant heat.

Our 2014 Prius uses a THERMOS-bottle in the cooling system loop to help retain coolant heat.

Actually, that was a feature on the Gen 2. Gen 3 runs coolant lines to the exhaust for faster warm-up.

http://hiwaay.net/~bzwilson/prius/pri_2010_050.jpg

I was wanting to test out this automotive insulating paint called nansulate on the engine, it supposedly gives you a r-11 insulation rating with 3 mil's of paint(or three thick layers of paint according to the manufacturer) I believe that having the heat held in the motor is best so that you can have the cooling systems for when it starts to overheat. Also if the coolant system isn't enough to get the heat out after insulating, I was planning on adding a oil cooler later anyway.

I've seen that before:

KaleCoAuto Radiator Insulator
$22.95

+1 on that, I run with two fire blankets all year with no safety issue. Just be careful not putting the blanket in the serpentine belt or radiator fan areas. This is something really easy and really cheap (10-15$). It helps retain heat when you plug the block heater. You can easily get 10°c more with this setup. When you drive it helps too.

You can also do a grille block which I think is best for isolating the radiator because it also helps the aero.