The most important thing you can do to prepare for a call to serve is to become a missionary long before you go on a mission.
Increase in your desire to serve God.
Begin to think as missionaries think.
Read what missionaries read.
Pray as missionaries pray.
Avoid the worldly influences that cause the Holy Ghost to withdraw.
Grow in confidence in recognizing and responding to spiritual promptings.
You are preparing for a lifetime of missionary service. Be a missionary at all times and in all places.