Most climates can accomodate some sort of fall or winter crop. An excellent way to replenish your soil in the off-season is to plant a crop such as buckwheat, sunflowers, or alfalfa in your empty beds. These plants enrich the soil with tons of nutrients to replenish what nutrients your fruits and vegetables consumed during the spring and summer. Once these plants have had a chance to grow, turn what's left of them into the soil and by spring, you will have nice rich soil again. Also, it is important to rotate crops in your beds so that the same types of vegetables are not always in the same spot in the garden. Different plants take from and give to the soil different nutrients, so it's best to shuffle things around for the healthiest soil possible.