If you do it every 30-50k miles there's no need to do a "flush". Drop the pan, replace the filter, bolt the pan back up with a new gasket and fill till full. That'll get anywhere from 35-50% of the old fluid out. If you really want to you can drop the pan again after 100 miles or so and repeat the process, only don't change the filter. That should be good for 75% of fluid replacement.