Now this is important! Next time you make cookies stand by your mixer and watch the dough during the butter/sugar stage. There will be a point in time (about 2 minutes in) when you see the color and texture change - it will be considerably lighter, and look fluffy. At that time, you can add your eggs (one at a time), then watch again, you will see the color get even lighter, and the texture even fluffier. Once that happens, you'll be ready to add the dry ingredients.