1. Look for shiny, heavy-gauge sheets with very low or no sides. Avoid dark cookie sheets, which may cause cookie bottoms to overbrown.
2. Use jelly-roll pans (15 x 10 x 1-inch baking pans) only for bar cookies. Other types of cookies won't bake evenly in a pan with sides. If you must use this type of pan for other cookies, turn it over and bake on the bottom.
3. Nonstick cookie sheets let you skip the greasing step. However, the dough may not spread as much, resulting in thicker, more cakelike cookies.
4. Insulated cookie sheets slow baking and tend to yield pale cookies with soft centers. You may have trouble when using insulated sheets for cookies that contain a proportion of butter, such as sugar-cookie cutouts, because the butter may melt out before the dough is set. Don't bake cookies on insulated cookie sheets long enough to brown the bottoms because the other parts of the cookies will be too dry. On the other hand, if your oven runs a bit hot or browns cookies rapidly, insulated cookie sheets may improve results.