Most cookies, to me, are too hard. This is the result, I believe, of adding too much flour to the dough. Measuring flour correctly is the first step. But I think there is just generally too much flour in cookie dough recipes.
I always reduce the flour amount by 1/4 cup for every 2 cups of flour called for in the recipe. Then, the second secret: chill the dough for at least 2 hours before you shape and bake. Chilling the dough helps it keep its shape when baking, compensating for the smaller amount of flour. These cookies will be a bit more fragile than you're used to, but I much prefer fragile cookies to tough and dry cookies!
Try my tips in my favorite recipe for Chocolate Chip Cookies.