When you know you're going to be absent, you need to make sure that you inform your students. Go over your expectations for them in terms of work and behavior and reiterate that all work must be turned in to the substitute. It is also a good idea to tell your fellow teachers that you will be gone. That way they can help the substitute if there are problems.
It is important to double-check with the substitute coordinator before your absence and make sure that a substitute has actually been scheduled for your class. There is nothing worse than not getting a substitute. When this occurs, your students will usually be farmed out to your fellow teachers. This not only destroys your planned lessons but disrupts other teachers' lessons as well.
Finally, make sure that you have left your lesson plans and your substitute folder in full view on your desktop. And if you aren't confident the substitute will be able to find the folder or that it may go missing, leave the information with the teacher next door.