Moss is a normal occurrence in our northwest climate. However if it's over taking your lawn, then it's a symptom of some other problems like:
1) Poor drainage- solve this by aerating and installing drainage if needed.
2) Poor soil- not easily solved but you can top dress with a good soil or compost over a period of time.
3) Lime- a rate of 50 lbs per 1000 sq. feet of lawn increases the sweetness of your soil
4) Improve sunlight- trim back or remove trees
5) Over seed with a shade tolerant type grass.
6) You may need to remove the grass and come up with a different plan for the area.
Spring is coming! I see the daffodils starting to poke through!