Behold, this is the promise of the Lord unto you, O ye my servants.
Wherefore, be of good cheer, and do not fear, for I the Lord am with you, and will stand by you; and ye shall bear record of me, even Jesus Christ, that I am the Son of the living God, that I was, that I am, and that I am to come.
—Doctrine & Covenants 68:5–6
You can prepare for a mission by being a great member of your ward (e.g., Attending all meetings, participating, making others feel welcome at church, etc.) Oftentimes the missionaries are relied on to keep meetings going, to step in when a teacher no-shows, and to keep their investigators as well as other members engaged. Your ability to do those things well comes most easily with practice.
Logan, Poland Warsaw Mission
One of the most practical things you can do is to study the Missionary Lessons in Chapter 3 of Preach My Gospel. Read and study the scriptures listed at the end of each principle as part of your daily scripture study. Practice making lesson plans and refer to the example short, medium, and full lesson plan outlines at the end of each lesson. Being familiar with what you will be teaching will really help you hit the ground running in the MTC and mission field!
Brett, England Leeds Mission
My suggestions are:
Tiffany, Scotland/Ireland Mission
I think that the best thing someone can do to prepare for a mission is learn how to work hard and be diligent. You should read your scriptures and have a testimony of the restored gospel.
Will, Chile Santiago West Mission