1) What things did you do in High School that you are glad you did - things that helped you be more ready for a mission?
* Go to seminary with the attitude that whatever the lesson was on, was something that God wanted me to hear that day and that there was always something to learn. Even if it was just scripture mastery quizzes. When I went I went with the intent to be edified and uplifted, seminary became much more personal and spiritual and it helped me to leave that building with a renewed testimony to choose the right and be a better person.
* Remember that your parents and leaders are "God's messengers" in a sense. God has placed these people in your life for a specific reason-- even your parents (I know that sounds funny). But they are there to help you choose the right and they want what is best for you, even if you sometimes don't realize or think it. Ask for their advice, learn from them, and let them become your best friends. Remember they are older and wiser!
*Don't forget to pray! For the majority of my time in high school, I would try to remember to pray every night. Then I remember multiple times when I just knew I needed help or I just felt alone. Maybe it was even before a basketball game-- Of course I knew that God wouldn't just magically at the end of the game change the score so that we would win, but he would give me the strength to play and the stamina to last the game. He would give me a clear mind to focus on the plays and guarding the person in front of me. He would help my body to function properly so that I could play. I can't say at the time (during high school), that I felt like my prayers were necessarily always answered (or at least I didn't notice), but I know that just that relationship that I developed with my Heavenly Father, helped me to know that I could find peace in just talking to him and knowing that he was listening. He didn't always take all my problems away or solve them for me, neither did he just magically make me change things that were wrong in my life, but he gave me the peace, comfort, assurance, guidance, and strength to do so myself. I may not have seen it then, and maybe you can't see the purpose of prayer in your life right now, but just DON'T FORGET TO PRAY! God is listening and your prayers will always in one way or another be answered, maybe not the way you planned or expected, but they will be answered.
2) Is there anything you wish you would have done different to prepare for a mission?