Choc chip cookies

How does one make fluffy choc chip cookies? I have made them for years and they always flatten out. I have tried everything possible but they still do not puff up. What is the secret?

Cookies made with butter will spread out more in the oven because it has a lower melting point than shortening or margerine. Try using shortening for at least half of the butter called for in the recipe.

Thanks so much for the tips and recipes- I cannot wait to try them.