If, when flat boxing, you have an extra flat box and handle of the same size, have a helper load mud in the second box. That way once you have exhausted the mud in one box, there is another full box always ready to go. When not filling the empty flat box, this helper can also be running a broad knife across the fresh mudded bands and butt joints too. This gets rid of lap marks, angle gaps, and air pock marks. Use a long handle to wipe ceiling flats from floor. Once you master this system it goes incredibly fast. It is best to have enough mud mixed to complete an entire level before beginning.