Use a room temperature egg. Ingredients at room temperature will combine more easily and completely than ingredients right from the fridge.
Be careful not to over bake the cookies. The cookies should still be soft in the middle when you take them out of the oven. They will firm up while they are cooling on the pan.
Store leftover cookies in an airtight container. The cookies will last up to 3 days at room temperature or up to a week in the fridge.