Stay morally clean: Stay clear of pornography or questionable movies and music, and date in groups. If you do have a girlfriend, discuss your goals and boundaries together and don’t get yourself into situations that could compromise those goals.
Prepare financially: This is something that may have started when you were a little boy. Regardless, though, of how long you’ve been saving, make becoming financially ready for your mission a high priority by working and saving your money. Even if your parents are financially prepared to help you on your mission, be sure to contribute what you can. Pay an honest tithe and then have faith that your temporal needs will be taken care of while you serve.
Strengthen you testimony: Remember all those SSAs (Standard Seminary Answers)? It may seem repetitive, but the most important things you can do to strengthen your testimony stay generally the same regardless of your state in life: pray sincerely and often, read and study your scriptures, faithfully attend your church meetings and keep the Sabbath day holy, and offer service to others. Stay steady in these areas and you will be prepared.
Increase your gospel understanding: Read the standard works and study them with the help of a study manual—not just the Book of Mormon, but also the Bible, Doctrine and Covenants, and Pearl of Great Price. Brainstorm questions you think an investigator might have and write them down; then search the scriptures for the answers.
Take a mission preparation class through your ward, university, or Institute. Also, prepare sincerely for your first trip to the temple. Speak about this experience beforehand with your parents, bishop, and/or other leaders, and consider reading President Packer’s book The Holy Temple. You can also take temple preparation classes in most wards—ask your bishop about them.
Practice: Bear your testimony as often as occasion will permit leading up to your departure. Go on splits with the missionaries in your area. Muster the courage to speak with non-member friends and neighbors about the gospel—bringing up the fact that you’ll soon be leaving to serve a mission can be a great spark to a gospel-centered discussion.