After my unfortunate experience with the packing tape and newspaper, I found an easier solution.
I found a roll of wide masking tape, and then located a strip of aluminium about 3 feet long, used to cover the edge of a linoleum floor. I ran the masking tape down the edge of this, to hold it on the 'edge' where the car stops being horizontal and starts being a vertical door/side.
This gives a nice straight line for the black undercoatings, which contrast against the red. I am doing a three foot section at a time for the final painting. Its easy once you've finished sanding and have cleaned off all the dust with a rag from the sides and bottom (I'm working outside while its still warm and sunny).
Your final result should look something like this:
Indoors you will probably want to control dust a lot better especially if you intend to paint indoors too.