I'm not a moderator but have been through it a few times now. To be prepared, you should have your child lists complete (sent by the local authority) and all of your books ready and easily accessible and know which evidence you would prefer to use. Teachers need to be released for the visit and a comfy room (with drinks and biscuits!) to moderate in. You should have read all of the documentation and be familiar with the interim assessment framework and how the process will work. If you do all of that, I think it creates a nice and calm atmosphere for working and sets a sense of professionalism.