Always use room temperature butter as it produces a smoother, lighter dough.

Beat butter and sugar together very well.  You can't over do it.

But don't over mix completed dough.  Simply stir with a wooden spoon until blended.

If dough seems too stiff or crumbling when forming cookies, stir in a couple of tbsp. of milk.

Test your recipe by baking 2 cookies before putting the batch in the oven.  If cookies spread out too thinly, the temperature may be too low or you may need to stir more flour into the dough. 

Use cool baking sheets.  On warm sheets, cookies spread too quickly.  To cool sheets between batches, run cold water over bottoms.