The idea is to have someone ready to step in when the time comes for you to move on. It's not enough to send out a general announcement. Spend time with each person who will be working closely with your successor. Explain why the change is happening and why are you confident in the abilities of the new boss. And be sure to describe what the change means to them.
Every job has one.
Spend a lot of time on personalities. Talk about the key people, how they like to receive information and their agendas, hidden or otherwise. Let your successor follow you around, observing exactly what you do every day. Throw in some reading you have found useful and follow up with question and answer sessions. Try to come up with some opportunities, things that already are in the pipeline, to give the new person a chance to start off with a bang.