Gently tap the cake pan against the kitchen counter a few times and swirl the cake pan before placing the cake in the oven so that any excess air pockets are removed.  Also, it's important that your oven has enough time to preheat so that you can place the cake in the oven right away.  Some air bubbles are common as the leavening agents are working.  The good news is that any imperfections will be covered with icing sugar!