Several factors could be contributing to this problem:
• Using too much grease on the cookie sheet. Grease sheets lightly or use parchment paper instead.
• Using warm cookie sheets. Be sure to cool sheets to room temperature between batches.
• Substituting butter in cookie recipes that call for shortening. Try half butter and half shortening to achieve a better result.
• Making cookies too big. Try making smaller-size cookies.
• Using warm cookie dough. Refrigerate—or even freeze dough balls—before baking.