President Thomas S. Monson, has declared, "Young men, I admonish you to prepare for service as a missionary." Then, "... sisters: while you do not have the same priesthood responsibility as do the young men to serve as full-time missionaries, you also make a valuable contribution as missionaries, and we welcome your service."
Prophets have made the importance of missionary work very clear. But once you've made the decision what can you do to prepare to serve faithfully and effectively?
The first thing President Monson mentions in his talk, "As We Meet Together Again," is to keep yourself clean, pure, and worthy. Second, maintain your health and strength. Make sure that you are taking care of yourself. As a missionary, you will have 30 minutes a day to exercise. Make those minutes count. Third, go to seminary or institute. There you will learn the basic doctrines of the Church. And last, but not least, study Preach My Gospel. Familiarize yourself with it's pages, it will help you be the most effective missionary you can be.
If you do these things faithfully, and to your best ability, you can become a great instrument in the Lord's hands.