When your cake turns out dry and crumbly, the natural inclination is to assume that you cooked it too long. But before you fault yourself, remember that there are many other factors that influence the moisture of a cake. Aside from using an oven thermometer to ensure your oven isn't sabotaging your efforts, there may be no better way to achieve a moist cake than to get a different recipe.
Checking for Doneness
An overcooked cake can be dry, but that doesn't mean the key to a moist cake is undercooking it. Follow the baking time instructed, but begin checking for doneness a few minutes before the minimum recommended baking time passes. You want to cook the cake until it is just done -- when the sides begin to pull away from the pan and a toothpick inserted in the center comes out clean, or with a few crumbs. If the toothpick comes out with streaks of batter, it is undercooked and, while certainly moist, would be difficult to slice and serve; put the cake back in the oven to continue baking and check again in a few minutes.
The Recipe Is Key
Recipes for moist, tender cakes are as much a matter of chemistry as they are the baker's taste. Recipes that begin by creaming butter and sugar produce cakes that are more moist than those that begin by whipping egg whites. Cakes that require bread flour are dense, moist and more tender than those that require cake flour, because bread flour has a higher gluten content; all-purpose flour contains less gluten than bread flour but more than cake flour, and lends itself more to dense, moist cakes than light ones. When searching for recipes, opt for those with oddball ingredients such as sour cream, buttermilk or cream cheese; the additional fat and tenderizing nature of these ingredients lend richness to a cake. Mayonnaise, another surprise ingredient, also contributes to a moist cake by way of additional fat.