What is the most important item a substitute teacher must have, in my opinion? The class list! Fortunately, where I sub I am always able to get one.
The key to to the school is also VERY helpful, if you have kids young enough to need recess.
Patience....you must have patience. Everything will not go perfectly. Find the child you can trust and ask him/her how the teacher does such and such. The teachers I sub for usually include the name/s of these kids in their lesson plans. Very helpful.
And pray for grace and peace.... :-)