Moss in your lawn is nearly a sure sign of four things:  Shade, moisture, alkaline soil, and infertile soil.  If you are of the mind set that a lawn should be grass and moss has no place in it, here are a few things to do.

Since moss will readily grow in soil that is too infertile for grass, one step in controlling moss is to amend your soil.  Addition of a lawn fertilizer or organic ammendments will enable the grass you desire to grow to do just that.  With the grass growing, the moss will not.

Moss also tolerates, if not thrives in acidic soil.  Besides improving fertility, you should also consider adding lime to the soil to raise the alkalinity.  While it is best to have a soil test done, the addition of lime will help reduce the presence of moss.

Moss also grows frequently best in shade.  Cutting away some lower limbs to let in more light promotes the growth of grass instead of moss.

Finally, areas that are chronically damp are frequently places where moss grows.  If you can build up this area or otherwise make provision for better drainage, it will help control the moss.