The key to this wonderful, subterranean environment is the undisturbed labyrinth of hyphae. Tilling and even simple hoeing can rip apart this delicate lace, which can then take months or even years to reestablish. When tilling is a yearly event, the fungus will never get a foothold. Too much synthetic fertilizer can also damage the fungus, as can pesticides and, of course, fungicides. Solarizing your soil (covering it with clear plastic to let the sun’s heat kill weeds and insects) will also destroy mycorrhizae.
Soils high in organic matter are where mycorrhizae flourish, so adding compost is one of the best ways to encourage the establishment and growth of the fungi. Rather than digging in organic matter, which will destroy the fungal net, lay the compost on top of the soil and let it naturally decay into the soil.
Mycorrhizal inoculants are available from several companies, but adding these to your soil is usually not necessary unless the soil microbe population has been damaged. In addition, many strains of VAM are plant specific, and what you buy may not be the right one. Often, the products will contain a mix of many different fungi in the hope that one will match your plant.