Some General Baking Tips

Back in the days when I took Wilton Course 1 there was a page in the back of the lesson plan with tips on how to bake a great cake. The idea is that your cake is the canvas on which you decorate, so it's important to start with a well-baked cake.

1. Preparing the cake pans: generously grease and flour the pans. Spread vegetable shortening evenly over the sides and bottom of the pan. Dust the inside surface of the pan with flour and tap out any excess. This is especially important when baking character cakes because you really need to be able to see the lines and indentations the pan imprints on the cake surface. 
2. Preheat the oven. Having your oven at the correct temperature is crucial. It may take as long as 10-15 minutes for some ovens to heat to baking temperature. Oven thermometers are an inexpensive way to ensure your oven's thermostat is accurate.
3. Measure liquids at eye level in standard measuring cups. 
4. Use large eggs.
5. Beat cake mix batters for the time and speed specified in the directions on the box. Overbeating can break down the cake structure, causing low volume. Underbeating produces a lumpy batter with ingredients not thoroughly blended.
6. Bake cake immediately after mixing the batter. 
7. Bake cake layers in the center of the oven. Allow at least an inch of space on all sides between pans to allow air to circulate. If using two oven racks, stagger the pans so that one cake isn't directly above the other.
8. Time baking accurately. Do not open the oven door before the minimum baking time has elapsed. This will cause heat to escape and may alter the oven temperature causing the cake to fall.
9. Cool cake in pan for 10 minutes on cooling grid or cake rack. Trying to remove the cake while it's still too hot may cause it to break or fall apart. If the cake stays in the pan for too long, it may not easily release from the bottom.
10. Cool cake completely before attempting to ice it. Brush any loose crumbs off of cake surface with a pastry brush. A cold cake is much easier to ice, so if at all possible chill the cake in the freezer first.

The nice thing about using boxed cake mixes is that they're pretty much foolproof. Baking from scratch can be a different game altogether, however. Some key things to keep in mind:

1. Use the freshest, best quality ingredients possible.
2. Make sure all ingredients are at room temperature. This includes the eggs and milk. Eggs will separate more easily while cold, but for best results allow them to warm to room temperature before using.
3. Measure ingredients carefully and accurately. The easiest cake recipes to follow give ingredient quantities by weight. Just be sure to stick to the same standard--no mixing metric and imperial measurements. When measuring ingredients by volume, pay very close attention to how the recipe is worded. For example, if it calls for 3 level cups of cake flour, it's best to spoon or pour the flour into the measuring cups and then level off the top with a knife. Whereas a "heaping" cup of flour is just that--there is no need to level off the top of the measuring cup. Also note whether dry ingredients should be sifted before or after measuring. 
4. Cream butter and sugar until light and fluffy. This is very important.
5. Gently coat nuts, raisins and chopped fruits in a little flour before adding them to your cake batter. This will help keep them suspended in the batter and not sink to the bottom.
6. Use the pan size and type called for in the recipe. If the pan is too small, the cake batter will over-flow during baking. The cake batter will over-bake and under-rise in a pan that is too big. Fill pans no more than 2/3 full.
7. Simple syrup is an excellent way to add moisture and flavor to cakes. Heat equal amounts of white granulated sugar and water in a saucepan until it comes to a rolling boil and all the sugar dissolves. Allow mixture to cool, then use as is or add your own favorite flavoring, extract, or liqueur. Apply to cake top and sides a with pastry brush or via fine-mist spray bottle.
8. When baking a chocolate cake, try dusting the pan with cocoa powder instead of flour. 

There are many more tricks and tips out there. It's impossible for any one person to know them all, so please feel free to add yours in the comments!