Read the recipe to the end and make sure you have all the ingredients. Measure out ingredients, from 4 cups of flour to 1/4 teaspoon of salt, and have everything at hand next to your mixer.
DON'T TRUST YOUR OVEN
Most ovens run a few degrees hot or cold. Invest in an oven thermometer (about $6) and adjust temperature accordingly. You also can have your oven recalibrated.
Always preheat your oven for at least 15 minutes before putting in cookies. Many ovens take longer; check your oven thermometer (see above) to make sure oven has come to the correct temperature.
MAKE EVERY PAN NONSTICK
Instead of greasing pans, line them with parchment paper or, better yet, buy a couple of silicone mats (Silpats); they can be reused hundreds of times and virtually eliminate the need to wash baking pans.
ROTATE FOR EVEN BAKING
If baking more than one pan at a time, rotate the trays in the oven after 5 minutes to promote even browning: Switch the positions of the top and bottom pans, and turn each one from back to front. Unless your oven is particularly even-heating, rotate even when baking only one pan.
TIMING IS EVERYTHING
Use a kitchen timer, and check cookies a few minutes before the recommended baking time.
Unless otherwise instructed, place baking sheets on wire cooling racks and let cookies come to room temperature before removing them. If you have used parchment paper or Silpats, cookies will lift right off. Make sure cookies are completely cooled before storing in airtight containers.