Our oldest daughter leaves to enter the MTC in less than two weeks, and she will be speaking in church today - along with one of our other daughters and me. I will ask them if I can post their talks after they have given them, but I want to share the outline of what I will say.
I am speaking on "What Advice I Would Give My Child if S/he was Going on a Mission". I will flesh out each piece of advice to match the overall time I have (and I will try to update this post afterward, if possible), but I want to share the overall structure of the talk while I have the chance.
1) Be yourself. The Church isn't calling "a missionary" to go to Berlin, Germany. It is calling you. There is someone there who needs more than just "a missionary. There is someone there who needs you - your unique perspective, your unorthodox views, your lack of inhibitions, your weird sense of humor - you.
You play the piano and the saxophone; don't try to play the piccolo.
2) Love unconditionally. Serve and help in whatever way you can, regardless of whether or not someone accepts the Restored Gospel completely and joins the Church. Your ultimate goal is not to convert everyone. The final result is out of your control to some degree, so do what you can: Honor agency, and love people.
3) Seek to understand before seeking to be understood. Learning someone's language is more than just learning how to understand and say the words they speak. It includes learning what they really mean and what they believe, as well.
4) Teach the Gospel, not culture.
5) Define success as it is defined in "Preach My Gospel", even if some people in your mission define it differently. The official definition of the Church, recorded in that manual, stresses effort, not numbers. Never forget that.
6) Pray as if everything depends on the Lord, but work as if everything depends on you. Always combine the two: prayer and personal effort. Take his yoke upon you - and remember a yoke joins two in order to balance strength and accomplish what one can't do alone.