Mechtech2, there may be a better compound out there... I don't know. I remember 25 years ago researching rustproofing for myself. I queried the experts at Chrysler when I worked there. These were engineers who best knew rust and rustproofing. They pointed me to the Texaco compound. I talked to the people who make the Polygard rustproofing. I was not impressed with the fact that it turns waxy and can harden with time, delaminate, and trap corrodants, making rust worse in the latter stages of the coating's life.
I find there are other products that emulate the formulation of Texaco rustproofing. Also, Texaco rustproofing is priced right... $65 for a 5-gal pail, which can do many applications. Once I find something better than Texaco rustproofing, I'll be sure to let everyone know. If anybody finds something better than Texaco rustproofing, let me know.
Keeping the car clean and clearing the drain holes are not all-important. They help, but the real reason for corrosion is that the metal is not sealed from the elements. Once paint is chipped from stone damage, or breached for some other reason, corrosion sets in, and there is no stopping it, unless the car never gets wet again. Rustproofing compound seals the metal from the corrosive elements.