You don't need to do anything, but if you don't everything gets chucked into the same box from the room that the box is in. So I would have a good look around to see what you don't want to take and chuck it out beforehand, and maybe box up the stuff that you want to box yourself.

They'll leave out anything you ask them to. It might be an idea to put that in a box somewhere and label it 'leave', in large letters. Otherwise, just get out of the way and allow them to get on with it. The great thing about having someone pack for you is that they don't dither and wonder and root through memories - they just get on with it. They packed up my house in Grove in four hours. Everything! They just did it.