I have spent most of today preparing for a group of third graders whose class I will be stepping into on Friday. Since I am scheduled to be there through December, I need to treat this class as if it's my own. I made goodie bags filled with a pencil, bookmark, small notepad, and a note from me. I organized my supplies and created my system to keep up with grading. Something else I did, that would be good for long-term and short-term subs alike, is create a class set of craft sticks. I chose fat colorful sticks, but any sticks will do. The great thing is they are very inexpensive at any craft store, and they are small so they store in a sub bag of tricks easily.
Use the sticks for anything from calling on students, to taking role, to choosing a line leader. It is always fair and keeps students listening since they never know when they might be called.
Another trick I use with these is to put them in the holder number side down. When I have chosen that stick, I put it back in number side up so I don't call on the same student frequently.
Many teachers I have seen have these in their classrooms already, but as a sub you never know if they will be there or you will able to find them. So it is an inexpensive, easy, and small-to-store thing for you to carry with you.