In Argentina the young men and women preparing to go to their missions go in 'mini-missions' they stay with the missionaries for approx. 2 weeks, they dress like missionaries and they follow the same rules that the missionaries follow. I heard great experiences about them and it really help them to know how to get along with their future companions.
"The single 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. Please notice that . . . I emphasized becoming rather than going. . . .
". . It is possible . . . to go on a mission and not become a missionary, and this is not what the Lord requires or what the Church needs."
(David A. Bednar, "Becoming a Missionary," Ensign, Nov. 2005, 45)
I really agree with what Joseph has said and quoted..
Just as you will have the advantage in the next life if you gain knowledge and understanding, so will it be for your missionary efforts. The sooner you start pulling out the missionary lessons, studying them, and learning the scriptures that confirm the lessons, the stronger your testimony and teaching ability will become.
Your missionary experiences can anchor your testimony for the rest of your life.
Here are a few things I wished I did BEFORE my mission: (not in order)
Read Preach My Gospel (this came out after my mission, for me it was the missionary discussions).
Start memorizing all the Scripture Mastery verses, and all other scripture verses in Preach My Gospel.
Read ALL the scriptures; Bible, Book of Mormon, Doctrine and Covenants, and Pearl of Great Price; cover to cover, every page, and every word!
On the churches internet site, lds.org, you can search for different subjects, like missionary work, read as many articles that you can.
The Gospel topic page is: Source 5
Talk to the missionaries in your area, ask them what you can do, also ask if they can give you a missionary lesson so you will become familiar on how to teach one. While you"re at it go with them, and help them, teaching someone the gospel.
Do your Home/Visiting Teaching, if your Bishop hasn't given you a Home/Visiting Teaching partner then ask that he give you one. This will help you to become comfortable teaching someone the gospel in his or her house.
Become familiar with the basics of the gospel: The Plan of Salvation; The Atonement of Jesus Christ; ect.
Learn how to cook, clean, do your laundry, FOR YOURSELF!
Start saving for your mission, not only for the $375 a month, but for the other expenses, like white shirts, a dark (I would suggest a black) suit, a journal, a bike (most missions expect you to buy a bike, if you are go to a bike riding area), a book bag. All of this can cost hundreds of dollars.
Learn how to budget money, remember that money donated for missionary work is the Lord's money and should be treated as such.
A mission to Alaska, and a mission near Death Valley, will have vastly different clothing requirements, so don't buy coats, or sunscreen until you get your mission call.
Talk to your Bishop, this is the most important one since he will tell you not only of physical preparations you will need but will tell you of spiritual preparations you will need to have before your mission.
Edited: haleray on 4th Dec, 2008 - 7:47pm
Haleray along with the other messages above gave excellent advice for anyone willing to serve the Lord. I just want to emphasize that having a love for the Lord before you serve will help you tremendously in the beginning months.
I agree with you LDS expert, most of this thread has focused on the physical aspect of getting ready for a mission, but the most part of getting ready is to love the Lord, and wanting to go on a mission. Fast and pray, ask your family to do the same, especially if your not sure on what to do.