In most cases, verbal permission is all you need, but there will be times where you might want the permission in writing, or where the property owner may be hesitant unless she or he has it in writing that you won't hold them liable if you are injured on their property.  At those times, it is good to have a paper form handy that you both can sign.  This is the form that we use - feel free to borrow any of our wording when creating your own.