One thing to be careful of are non-margarine ‘spreads’. Products labeled as margarines are required to have very similar fat/water ratios as butter, and will behave similarly in baking recipes. Most of the butter substitutes you see nowadays (especially now that trans-fats are viewed as bad or worse than saturated fats) have a lower fat and higher water content than butter or margarine, and should absolutely not be substituted for them in baking. Eg. Country Crock, Smart Balance, I Sure As Fucking Hell Can Believe It’s Not Butter, etc.
A second for this tip. They’ll cook for at least two more minutes outside the oven. I’ve found it to be essential to getting soft, chewy cookies.
“Three chips for sister Marsha” is available on youtube. Be sure to watch part 2, it gets into the science of chewy cookies.