Allow your missionaries to grow up with minimal interference from you or dad. This means you shouldn't call the Mission Office to find out if he/she is sleeping on clean sheets, eating properly, etc. There are some concerns the Mission President should know about your family, such as: death of a family member, serious illnesses (the kind that require surgeries and hospitalization), etc. Not if someone breaks a bone or needs stitches—we mean SERIOUS stuff. And, worthiness issues you might be concerned about with your missionary.
Encourage your missionary to abstain from participating in Mission Gossip. We need to let our sons/daughters know that this type of behavior is not conducive to the Spirit. Be a peacemaker means being quiet about information that shouldn't be spread around. Make certain you do not spread gossip, either! Learn to "zip it" if something is shared to you, either through an email or in person. You do not need to see your email posted on a missionary bulletin board, and neither does your son or daughter.
Show compassion over your missionary's struggles, but let them figure things out on their own. You can offer a few scriptures or comments, and let them know you are praying for them, but allow them to grow spiritually and develop inner strength.
Keep their names on the Temple Prayer Roll, and attend the temple often.
Get a copy of the "Missionary Bible" the rule-book for missionaries (Distribution Centers). Read it and know the rules, and let your missionary know that you've read it. An ounce of prevention is worth a pound of cure.
Write letters that are encouraging, uplifting, and upbeat. Talk about your own spiritual experiences. Share your notes and thoughts from Sunday School lessons, Sacrament meeting talks, and spiritual impressions. Share faith-building stories from your ancestors, and conversion stories. Give them fuel and don't douse their fire!