The short answer: not much and sometimes nothing.
We will do most of the preparation for you. We do things like cleaning the walls, removing outlet covers, masking, and taping are all part of what we do. We do suggest you take down the wall decorating stuff like picture frames and clocks etc., and store it safely as you may want to take extra care in boxing and bubble-wrapping delicate items. This is not labor intensive and you may enjoy spending some extra time with your items. Extra pre-painting vacuuming if you have pets can also be helpful.