Baking is not very forgiving when it comes to estimation. Having a good understanding of measuring technique will yield better results in the kitchen. Small variances in quantities of ingredients can have a major impact on the final texture, structure and overall quality of the desired product. All of the ingredients used in baking (such as flour, sugar, eggs, butter, leavening agents, etc.) have key functions. So, precision is your friend here!
Always measure liquid ingredients in glass volumetric measuring cups. Likewise, measure dry ingredients in plastic or metal measuring cups or spoons (also called dry measures). These tools are typically not interchangeable. Using the "scoop, sweep & level" method is ideal for dry ingredients, unless otherwise specified in the recipe. To do this, draw your measuring cup through the ingredient, such as flour or white sugar, to gather a heaped amount. Be generous at this stage. Then, use a straight edge, such as the blunt side of a knife, to scrape off excess by dragging it across the surface of the measuring cup and creating a level surface. Some recipes may require you to spoon the ingredient into the measuring vessel in order to create a lighter product. The message to take home from all of this is: do not pack it down! Never tap the measuring cup, as this causes the ingredient to settle and become more dense.
The exception to these rules (there's always an exception *sigh*) is brown sugar. Most, if not all, recipes call for packed brown sugar. In this case it is important to press and pack brown sugar into the measuring cup until it is level.