Avoid the use of pesticides and fungicides (including using neem oil as a soil drench). Avoid tilling or compacting the soil, which breaks up beneficial soil structure.
Don’t overfertilize. Grow a cover crop in the off season, or when you don’t have plans for a bed. (My favorites are Austrian winter peas and fava beans, which give you delicious bonus harvests year-round.)
Grow anything, as mycorrhizae need a root system to thrive. That means you should keep some plants growing in your garden over winter, especially those with extensive root systems.
Strawberries, garlic, onions, chives, and other perennial herbs and perennial vegetables are all good choices for overwintering. If you live in a mild climate, you can keep many more plants going all winter.
The good news is, you can always “start over” and rebuild the mycorrhizal network by using special inoculants or soil amendments that contain them.