2. If possible, have your child live away from home for a period before he or she serves a mission. The experience of sharing space, time and resources should not be new to a missionary who will have a companion with him or her for 24 hours, 7 days a week during the mission. You can tell those who have had the prior expereince and those who have not. So can their companions!
3. Teach your child that a mission is about serving others and not others serving them. Service requires acceptance, patience and sacrifice. No matter where your child serves it will be different than "home". Different does not mean worse or better...it means not the same. If your child does not appreciate the people he or she serves for who they are, they will not accept your child for who he or she is trying to make them become.
4. Prepare your child for the fact that a mission can be hard at times. It is called missionary "work" for a reason. People who have learned the virtue of committment, the skill of setting goals and have the experience of working through obstacles and solving problems will find a mission a rewarding experience. (Work and fun are not mutually exclusive) Those who have not had to develope these qualities will struggle until they do develope them.
5. Be supportive of your child's decision to serve a mission. Every child wants the support of his or her parents. Before and during the mission when you communicate with your child be supportive. A negative email or phone call, or one that reminds your child of everything that is going on that he or she is missing may be informative but it is not supportive. Even though events may have already occurred or there is nothing your child could do to change circumstances, he or she will worry about them if you bring them up.
It is a privilege to serve a mission...it is a blessing to serve a second mission from the vantage point of the Mission President. I wish I had been as prepared when I served my mission at age 19 as the young men and women who are currently serving. I am sure every Mission President has his own advice to offer and my thoughts may change the longer I am here, but for now, that is my unsolicited, solicited advice!