I have also been interested in the sugar starch method, but didn't know the proper amount.  

Regular laundry starch only works for about one wearing -  then the petticoat is back to its normal limpness. Spray starch is even more useless. I have had some luck with fabric stiffener which is made for draperies and doilies. Someone once told me clear liquid floor wax would work. 

I've asked two gals who run 50s sites about sugar starching; we will see what they have to say. Meanwhile, one gal had this fun paragraph on her site. 

"I remember the top of my closet stuffed with crinolines--the flouncy net slips that served to keep your circle skirt standing out. Does anyone remember washing them in "sugar water" in a vain attempt to keep them stiff? A new crinoline was required every three months or so--but the old ones weren't discarded, just added to the bulk of the new ones! I can remember wearing three crinolines at a time. (Barbara)

As far as I know sugar starching is only really effectual on paper nylon (parchment) petticoats. I have tried it on nylon net and crystal petticoats, but it is only partially effectual. These petticoats are very stiff to start with, but the stiffness only lasts for a short time, whereas with with paper nylon it lasts much longer. The procedure that I have used is to dissolve a 1 kg bag of sugar in a bucket 10 litres/2 gallons of warm water. Then dunk the petticoat under the water until it is fully immersed. Leave to soak for about half an hour, or until the petticoat is thoroughly wetted (with paper nylon sometimes it takes longer). Remove the petticoat, and hang up to drip dry over a bath. Before fully dry, try and arrange the petticoat to its greatest fullness. 

You can also use laundry starch on calico and cambric petticoats. That works really well, and cambric particularly stays really stiff for a long time. In this case drip drying does work, but ironing gets even better results. Repeated starching and ironing can get really very stiff results, such that the petticoats really crackle. Also good for cambric aprons and pinafores, which should be popular with the LG enthusiasts. Spray starch also works, but is not as effective as real laundry starch. The latter is more difficult to find these days (at least in the UK), but some shops still keep it.

The very idea is appalling! What happens if the day is warm? When perspiration makes an appearance?! I cannot believe that there was not some proprietary product which was designed for just this job back in the 50's/60's.

I've tried several starch-based products which just do not work on petticoat materials. While there are nylon rejuvenators such as "Fabulon" in Europe, there is nothing that does the job in revitalising a frequently-laundered petticoat.

How was the original material stiffened? Surely the right approach is to use whatever the original product manufacturers used back then.

I am fashion historian and not only theoretical, but even practical. As I adore to see my girlfriends in starched crinolines and they don't know anything about the try I had some weeks ago. I put boiling water in to a plastic sink and then a lot of sugar. Put the crinoline in it, spread the crinoline at the floor, and let it dry. Couldn't be better! The classical way is still the best!