just to add substantiation to the posts re: covering the grill... 
here's what i do in the tundra up here, apologies to the old timers 
that recall this thread last year.

1.  a BIG o' piece of cardboard in front of the lower and upper 
grills.  this does several things:
	a)  reduces the 'passive' air infiltration rate in the (very) cold 
air intake.  this allows the heater core to bring the output air temp 
higher, albiet at a lower total volume.  it IS warmer inside, however.
	b)  covers the radiator so that the thermostat sends more coolant up 
front to be cooled.  since the heater hoses run parallel with the 
main coolant hoses, they stay warmer.
	c)  marks your vehicle as a 'local' so that marauding highway 
predators don't bother you.  :)  (always worked so far)

2.  boil out the heater core if you've never done it to the vehicle.  
this has made a dramatic difference in both the '85 and '87 i've had. 
 not wanting to run the cleaning acids thru the questionable heads i 
disconnect the heater hoses and circulate boiling radiator cleaner 
thru the heater core only with a Rube Goldberg conglomeration of 
circulating pumps, boiling kegs, and a propane burner.  it works.

3.  add a booster pump to the heater loop.  i did this in the '85 but 
not in the '87.  i used a Par-Jabsco unit but the seal always weeped 
a little anti-freeze (irrascable stuff).  Mercedes uses a similar 
setup so one could scavenge a pump from a higher end Mercedes.  In 
fact, if someone has one laying around, i will buy it.  with the pump 
in the heater loop, i could sit at idle and recieve full heater 
output.  it was great.

4.  don't run with the heater *air* levers wide open at highway 
speed.  the heater core is easily 'over-run' with cold air.

5.  ensure your anti-freeze mixture does not exceed 60% ethylene 
glycol.  heater transfer efficiency is reduced with e.g. proportions 
above that.

6.  if all the above fails, drive a Dodge minivan!  (sorry for the 
blasphemy but i'm jealous of our friends' heat in their vans)

-dan