this depends on what type of decay you are asking about. the decay of tissue, or the decay of elements. elemental decay is the process in which an element (carbon, hydrogen, uranium, etc...) loses atoms, and is unpreventable. the decay of tissue is a process where decomposers such as scavengers (vultures), and bacteria eat away at the once, living matter. the only way to prevent this is to keep the composers away from the "thing."
They would use mummification.
By freezing it, salting it, drying it, pickling it, irradiating it, canning it and eating it now.
*wrappedMummies are wrapped in a hundred yards of linen to prevent the body from decaying.
Decaying is a slow process. It can take years.
vegetation that is decaying
The Egyptians believed that the bodies of the dead had to be preserved for the next life, and so they mummified(embalmed and dried) corpses to prevent them from decaying.
To prevent small mushrooms from growing in your potted plants, avoid overwatering, improve air circulation, and remove any decaying organic matter from the soil.
Foodsaver bags, if used correctly, will help delay or prevent decaying of vegetables. As long as they are sealed correctly, they prevent air that speeds decay from being trapped in with the food.
To prevent mushrooms from growing in your monstera plant, ensure the soil is well-draining, avoid overwatering, provide good air circulation, and remove any decaying organic matter promptly.
A decaying dead body
To prevent your houseplant from growing mushrooms, avoid overwatering the plant and ensure proper drainage in the soil. Additionally, remove any decaying plant matter or debris from the soil surface to discourage mushroom growth.
To prevent succulent mold, avoid overwatering your plants, provide good air circulation, and ensure they are in well-draining soil. Additionally, remove any dead or decaying plant material promptly.