As a trainer on my mission, I often talked with other trainers about how training was going. We would share ideas and talk about how our missionaries were doing. Oftentimes 1-2 trainers per flight group would say something like, "It is super easy training this elder. He came pre-trained," meaning he already understood how to be a missionary. For him, his home was the MTC.
Most people think that when you enter the MTC, you magically gain a testimony and a deep understanding of the gospel and how to do missionary work. You don't. You need to learn it. You have two options: learn it before you go, so you can be the most effective missionary possible, or you can procrastinate until you enter the MTC.
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After asking many missionaries (elders, sisters, and mission presidents), here are the top 7 things to do to come "pre-trained" to your mission:
1st: Gain a personal testimony of Jesus Christ and His atoning sacrifice.
You will be spending the next 18-36 months testifying of Jesus Christ and His Atonement. Develop a testimony now. Study His words and teachings. Strive to develop stronger faith in Him. Do all you can to learn of Him, and use His Atonement, for it will be your job to help others to do the same.