The first thing you can do to keep your sheet cake moist is to ensure your oven is heating to the correct temperature with an oven thermometer. You may be baking your cake in a hotter oven than expected. Secondly, do not over-mix your ingredients, as this can cause your cake to dry out during baking.   Finally, you can brush your cake with a soak. A soak is made from equal parts sugar and water and sometimes includes different flavoring or extracts. Brushing this on your cake will ensure that your cake is moist.