If your cookies are coming out flat, it could be one of several problems.  First, make sure your leavening agents are fresh - baking soda and baking powder - old ingredients yield bad results.  If your ingredients are fresh and you're still getting flat cookies, make sure you're using the correct measurements of all ingredients.  If your recipe calls for butter, you can substitute butter flavored shortening and that will usually yield a higher cookie.  Lastly, if all else fails, refrigerate your dough for an hour.  You may have to bake a little longer than the recipe calls for, but it will typically give you a higher cookie.