Avoid pressing hard and working over the same wet area repeatedly. This can cause the brush to lift already applied paint or result in color variations. Keep your touch light and assess the need for an additional coat only once the surface has dried.
Do not switch paint batches in the middle of a wall. If you are running out of paint, prepare a new batch before the old one is completely finished. Combine the new paint with the remaining batch and mix thoroughly. The safest approach is to prepare the amount needed for the entire wall before starting.