It is important to know what you want from a visit before inviting your author/illustrator.  Every author/illustrator is different. One may tell lots of jokes and have them rolling in the aisles; another may be softly spoken yet still have students hanging on every word.  One may be happy to talk to a hall full of children, another insist on class sizes no larger than thirty, another may prefer the intense workshop with only a dozen motivated students.  

Generally the more well known an author/illustrator is, the more difficult it is to book them.  They frequently limit the number of visits they do each year, to enable them time to write/illustrate.  Some also have higher fees.

If costs or distance are a problem you may like to consider a virtual visit.  Some authors/illustrators are willing to communicate with students via email or in chat rooms.  Some Education Departments have safe chat rooms available for such sessions. Education Queensland has tools in the Learningplace where teachers can run online projects with forums and chat rooms.

(Many thanks to Kate Veitch for granting permission to use material from her book, Real live writers - how to host a  successful author visit. National Book Council, 1995 in the Guidelines for Visits section of the website.)