This Sunday's paper (Salt Lake Tribune) had a whole section on holiday cookies plus lots of baking and party tips. Am I ever excited to try them! Here's one article that I thought looked helpful.
"Cookies are some of the simplest items to bake. But failing to use the right ingredients or baking techniques can mean the difference between an ordinary cookie and a great one.
Here are some things to ponder:
Butter vs. shortening » The best thing about butter: It adds flavor. It also adds fat, which creates tenderness. But butter can melt quickly and create a flat cookie. Shortening provides significantly less flavor, but has a higher melting point, so cookies come out puffy. Cookies made with butter will maintain their shape simply by chilling the dough before baking.
White vs. brown sugar » White granulated sugar is the most commonly used sweetener because it adds no additional flavors. It makes cookies brown and helps them stay crisp. Brown sugar, which is white sugar mixed with molasses, adds an additional layer of flavor. The acid in the molasses helps eggs to set up and cookies are more likely to hold their shape. Brown sugar also helps keep cookies moist and soft. So don't use brown sugar in crispy cookies.
Unbleached vs. bleached all-purpose flour » Unbleached all-purpose flour has a higher protein content. Protein prevents crumbly cookies and contributes to browning and chewiness. Bleached flours, with their lower protein content, create tender, puffier cookies, but they will be pale.
Creaming » Beating the sugar and butter together until creamy is the first step in making many cookies. But overbeating will add bubbles to the batter. Air bubbles will make the batter spread and will result in flat cookies.
Altitude » Cookies are not affected by altitude as much as cakes, but the batter can spread at certain elevations, creating thin, tough cookies. If this is a problem, raise the oven temperature 15 to 25 degrees. Reducing the baking time also can help. Cooks can add a little bit of flour or reduce the sugar to slow the spread. Another option is to reduce the amount of leavening (baking powder, baking soda or cream of tartar).
Source: The Art & Soul of Baking by Cindy Mushet; and Pie in the Sky: Successful Baking at High Altitude, by Susan Purdy"