I always felt that wishing people a Merry Christmas sometimes backfires because not everyone celebrates Christmas and some people may not be able to afford to celebrate. So I spread Christmas cheer with 'A smile and a happy greeting' so that I can give them at least a little sunshine in their day.
This may sound self-serving and I guess it is, but I have found the best way to spread Christmas cheer is to give to others. We have a tree at the local grocery store for the nursing home. People have things they wish for, sweat pants, magazine subscriptions etc. We try to do a couple each year--it really feels good to do so.