You'll want to do a weekly rotation. Start with the valves stems in the 12:00 position and then move them clockwise 1/8 of a turn each week for the entire winter. When you put the bike away, set the tires to storage pressure as detailed below.

Try to rotate them on the same day every week for best results.

Every 8 weeks, (one full rotation) you will want to refresh the air. Let all the air out of the tire and then squeeze the tire against the rim to expel as much of the remaining air as possible. It is OK if a little bit is left in.

Then inflate to storage pressure. This will be the rated pressure from the side of the tire less 20%. So a tire that has a max pressure of 100psi should be stored at 80psi.

When it's time to ride again, remove the storage air and inflate to full pressure for the riding season.