The best way to keep a cake moist is to stop all of the moisture from evaporating in the first place. To do this, I wrap my cakes in plastic wrap and stick them in the freezer or refrigerator as soon as the pan is cool enough to handle (about 5-10 minutes after it comes out of the oven). Doing this locks in all of the moisture that would otherwise evaporate as the cakes cool on your counter. Another way to moisten your cakes, should they end up on the dry side, is to brush them with a simple syrup right before frosting.