I recently baked some cookies, and attempted to make them like my mother did. But they turned out really flat (about half the thickness than what was desired). My mother cannot recall what variations she may have done when I was a kid, and told me to just follow the recipe.

My question is -- When making basic cookies (flour, sugar, eggs, baking soda, vanilla, margarine, etc) what variations should be tried to increase volume on the cookie and perhaps make them a bit softer (softer and thicker desired)?

Additional questions if possible--
What effect would 3 eggs have instead of required 2?
What if I doubled the baking soda (2 teaspons instead of 1)?
What if I used butter instead of margarine (or shortening)?

Or what other suggestions can be made?

Thanks in advance for any help!!!
~Patrick