Mow your lawn with a mulching mower or regular mower with no bag attached. Leave the grass clippings on the lawn. The clippings will attract worms as the plant matter begins to decay.
Keep your lawn and entire yard free of chemicals. Do not add fertilizers or anything chemically-based to "enhance" the lawn. These things are harmful to worms and wildlife.
Sprinkle a little compost in your yard or better yet, garden areas or spots that have little to no soil. Add fruit and vegetable peels from time to time as these serve as worm food. (If the worms don't find them, birds and squirrels will.) Coffee grounds also help attract worms to the soil.
Moisten the lawn in the evenings, especially when grass clippings and other dead plant matter are present.
Place a layer of mulch over any garden areas to help keep soil damp. Worms in garden and plant areas can work their way into your yard as well.