I have not tried the plastic bag method for removing tires but it certainly works great for putting them on.
There is an article in the Mar/Apr 1997 Restorer on removing tires from wheels which prescribes leaving the wheels and tires on the car.
Here is what worked for me (in part learned from the restorer article). After letting all the air out and removing the valve, let the tire and wheel sit in the hot sun to heat it up. Use a box similar to what is shown on the mantiques site. Put the wheel on the box and step on the tire to break the bead from the rim flange on both sides. With the wheel upright, on the bottom squeeze the tire beads together working them into the well in the center of the rim. Put the wheel back on the box with the back of the wheel facing up. Wrap a tire iron (s) with a rag and use it on the other side of the wheel (from the beads pushed into the well) to work the tire bead over rim flange. I suppose a sprinkle of talcum powder would help but usually it doesn't take any effort. Grab the released portion of the tire and tug the rest of the tire off the rim. Once one side of the tire is off I can usually pull the other side off without the tire iron. If you are lucky you may not need the tire iron(s).
Hope this helps.