Talk it up. The more excited you are, the more excited the kids will be. And when the kids are excited, any message the author gives them will really hit home. There are a few other things you should do for the author in advance of the visit. Discuss what time the first assembly will be, how many assemblies will be given, how long they will be, and how many students will attend each one.
Some authors prefer to talk to the whole school at once, others prefer intimate groups.
Some will only do their presentation once or twice in a day. Certainly don't ask an author to do it more than four times. It's impossible to keep the energy level up. Discuss the grade range. Some authors give a presentation that sails over the heads of grades K-2.