I take it it's not just in the middle you want your cakes to be moist? :-)
Many things come into play here, first of all the recipe used - some recipes just seem to make for dryer cakes, so make sure you experiment to find a good recipe.
Over-baking is the number one enemy of moistness, so make sure you don't bake your cake too long or at too high a temperature. Again, you'll have to experiment to find the temp and cooking time that yields the best result.