Stop using pesticides and insecticides on your garden.The chemicals in these sprays last for several years, making the soil poisonous for the earthworms.
Stop keeping your garden neat and tidy and let the leaves fall where they may. This is like putting out a feast for the worms. Clutter like weeds, leaves and grass clippings should remain on the ground so as to attract the earthworms.
Add organic waste and kitchen scraps to your soil. This is not only food for the earthworms but also regulates the temperature of the soil.
Water the plants and the soil regularly. The tunnels that they have created call for water retention. The earthworm breathe through their skin, which must remain moist but not too wet or else they suffocate. Hence, the soil must remain moist or else the earthworms will die.
Stop it with the digging. Just keep the digging of your garden to as less as possible, just enough to get the new plants in. Let the earthworms do all the soil spinning and whirling.