Moss can be found in patches of lawn where it grows better than grass for a variety of reasons. Poor soil quality, drought, a grass variety that isn't strong enough, too much shade or turf disease all can encourage moss growth in lawns. Fixing the problems that led to moss growth will help prevent the moss from growing in the future.
Getting rid of the moss invading your lawn is the first step to controlling it. Moss-killing products such as Moss-Out, Moss-Kil, Rid-Moss, or other products that are labeled for moss control will work well. They should contain ferrous sulfate or ferrous aluminum sulfate. These are safe for use on lawns and will even help strengthen the grass while getting rid of the moss. The best time to kill moss is in the late winter or early spring, before it has a chance to put on new growth.
After killing moss, dethatch the grass, raking out the dead moss along with weak, old or dead grass. Getting any thatch out of the lawn will help it grow more vigorously and keep moss down. Doing this helps air and water circulate more freely, reducing any drought stress and getting more nutrients to the grass roots.
Improve lawn conditions for the grass to favor grass growth instead of moss growth. Moss likes shady, damp areas that are poorly drained. Soil aeration can help with soil compaction and bad drainage, and thinning overhead tree limbs will help let in more light to shady areas where moss thrives. Soil pH is another factor to consider. Moss grows in more acidic areas, so adding dolomitic or agricultural lime at a rate of 20 to 30 pounds per thousand square feet of lawn will help create good conditions for grass to grow instead. The lime also will add calcium to the soil, encouraging grass to grow.
Moss grows best where it can outcompete grasses not well suited to cold temperatures. Ryegrass mixed with fescue is a tough, sturdy, cool-season grass that will keep moss from taking over your lawn in cool temperatures, and it grows in shade. If you are in a warm climate, plant a mixed lawn of ryegrass and Bermuda grass for year-round hardy grass that will strangle any moss. After dethatching, seed any bare areas with the ryegrass and dress the seed with a thin layer, about 1/4 inch, of loose soil, potting mix or sand, whichever is best as a soil amendment for your lawn. Water carefully but regularly until seedlings sprout.
Over-watering and over-mowing are two major contributing practices that actually cause moss to grow. Water lawns only when they really need it, instead of adhering to a watering schedule or a timed watering system. Mow grass at the top of its recommended height; don't remove more than a third of the top growth at any one time or the grass will continually be forced to put its growth into leafing instead of rooting.