With winter approaching I've been thinking the same thing. I don't think you can get real efficient, but here's my plan, for what it's worth.
I'm currently building a 2 racks. One is a quad rack for 4 55g tanks, the other is for 6 29's. I plan on putting 3/4" foam board on the bottom of all tanks. I also plan on putting plastic reflective insultation on the sides and back of every tank.
There's really nothing that can be done to retain the heat on the face or the top of the tanks, but I guess this will have to do.
Also, my "fishroom" is actually a spare bedroom since I don't have a basement. At night I'm going to put a 1500 watt heater in the room and close the door. I have no idea if raising the room temp will reduce the on/off time of the tanks heaters, but 1500 watts isn't really that much...
Brian