Fat, eggs, and liquid (eggs, butter, milk, etc.) should generally be used at room temperature. Cold ingredients could cause the batter to curdle.
When combining butter and sugar, take your time to cream them together -- beat or cream together for at least 5 minutes. This helps to ensure a lighter cake, as it adds tiny air pockets to the batter. Sugar granules shouldn't be visible, but you can still feel them if you rub a bit of the mixture between your fingers.
Measure flour and other dry ingredients exactly. Use a knife or other flat surface to level off dry ingredients in a measuring cup or spoon.
And don't skip the sifting! This step helps to add air and ensures that all dry ingredients are properly combined. If you don't have a sifter, you can use a wire mesh strainer.
When baking foam cakes like angel food cake, take great care when adding the flour to the beaten egg whites. Fold the flour in three stages, very gently so that you don't lose the airy volume of the egg whites -- this is what keeps the cake so light.
Generally, the cake batter should fill the pan by at least 1/2 and not more than 2/3, unless otherwise instructed.
Try to bake the cake in the middle of the oven and, if baking more than one cake at a time, don't crowd them together, or they will bake unevenly.
For crumb-free slices, run your knife under hot water and dry it before cutting into the cake.
Always wait for cakes to cool completely before frosting. Even the slightest warmth from a cake can quickly turn your frosting or icing into a mess.
Brush your cooled cake with a pastry brush (or your fingers if you don't have one) to remove excess crumbs.
Begin the frosting process by first applying a very thin layer of frosting to the cake. This is known as a "crumb coat." It will help to seal in the crumbs, and you can further help this process by refrigerating the cake for an hour or so at this point. The frosting will harden slightly and really hold in the crumbs. Now, you can frost as you normally would, starting with the top and finishing with the sides.
For a truly perfect presentation, wipe the spatula clean each time you swipe frosting onto the cake. You may want to spread it on smoothly for a clean finish, or you may opt to swirl it decoratively around the cake.