One piece of advice I always give prospective missionaries is to consider doing something that changed my mission: wake up early. I started off waking 30 minutes early, then slowly worked my way earlier. I usually woke up at 5:30am or 5am, to give myself even more personal study time. I tried earlier than 5 during a transfer, but it beat me up, so I made 5am my limit. It was hard, but so rewarding, life-changing even. Ironically, I felt like I had more energy than the missionaries who "slept in" 'til 6:30. My peers were so inspired by my actions that I usually got the whole zone to do it wherever I went, because the rewards were so amazing. In fact, the entire time, I only had one companion that wouldn't do it with me (he was struggling, lazy, wanted to go home, there for the wrong reasons, etc.) With just 60 minutes more study per day, you can get 728 more hours for your whole mission of study, opportunity to feel spirit, etc. My mission president allowed us to read any gospel book, so I read everything I could get my hands on, with an emphasis on the early brethren. Of course, you'll have plenty of time to study Preach My Gospel, to perfect your teaching, but make sure you work on that as well.
While you wait for your call, yes, I do agree with what has been said: strengthening your testimony of Christ, studying Preach My Gospel, etc. But since you have even more time (you can only study PMG in one sitting for so long), I say seek the early brethren. Find material that lights a fire underneath you. The early brethren did that for me. Reading their history, their doctrines, etc. It strengthened my testimony of Christ, gave me such a confident spirit, energy, and really inspired me to teach with the power of the Spirit. In addition, they shared enough meat for me to chew on that got my mind enlightened, my heart expanded, and my spirit filled. First books that really opened my eyes to the depth, power, and amazement of the Gospel were Key to the Science of Theology and The Seer (though it contains some speculation and a few false doctrines concerning Adam and the progression of Gods). I developed a love and unquenchable thirst for the spirit and power and knowledge of God. When I wasn't reading my scriptures, I was reading from the early brethren. I also really enjoyed the straightforward style of Joseph Fielding Smith and Bruce R. McConkie, though I may not agree with everything. Doctrines of Salvation was a popular book in our mission, and I really loved it.
I say, seek out the best books as the D&C says. Find prophets who speak to your soul. For me, it was many of the early brethren, and the modern fire/brimstone ones. Read everything you can now (in addition to your normal PMG/BoM study). You might get a mission prez who is very strict about not reading anything other than the typical books. That's unfortunate, if that's the case. Because reading other books was the catalyst that made me the missionary I was, and it changed me forever. I know many would disagree with my waking up early advice. "You need your sleep!" "It's against mission rules!" "If it was better for you, then it would be the standard mission rule!" I say poppycock, and I saw firsthand in me and tons of others in my mission, that it was inspiring and life-changing. You can take less sleep than you realize. 7 hours is plenty. Train your body, exercise, eat right, so you don't "need" the 8 hours. It all started when 3 months into my mission, my new senior companion was waking up early. I asked, "Why in the world do you wake up even earlier than 6:30?!" I thought he was crazy. He said, "I'm a convert. I only joined the Church 3 years ago. Everyone else has had their whole lives to learn this stuff. I feel like I need to make up for lost time." What was so amazing, is that this guy knew more doctrine than any other missionary I had met. Only 3 years member, and he was more stalwart, more knowledgeable, more inspiring than any other missionary I knew. It inspired me to also wake up with him, and I never stopped. The results were life-changing. Those early mornings filled me with very special and spiritual experiences. So, try it out at least.