Over mixing is a common problem when making cookies, especially if you have a stand mixer. As a child, I always used a large bowl and a hand mixer, so over doing it was never a problem! Now, however, I have a great Kitchen Aid appliance, which makes my life much easier, but also makes it easier to over mix my cookie dough.
The gluten in flour creates a strong network when moistened and mixed well. It gives food a chewy, elastic quality. It also gives food substance and strength, and helps it keep its shape. This is why bread must be kneaded prior to baking.
With cookies, however, elasticity and strength are not a good thing. Cookies, for the most part, should be soft and chewy. They should be more cake-like than bread-like. Therefore, when you mix cookie dough, be careful! If there is a fair amount of mixing to be done (and there rarely is), do it before you add the flour. Once flour is added to the dough and moistened, only mix enough to fully incorporate the flour, and then stop. Any additional mixing can make your cookies turn out dry and tough. This little secret can give you the softest and moistest cookies on the block!