So as a returned missionary, I often think of things I wish I had learned, or was taught before I went on my mission, or even earlier on my mission. These things would have changed how I taught, and my approach to my missionary work.
1. Learn how you learn and how others learn, then teach them how they learn.
Studies show that around 60-65% of people learn visually. I never connected this. I always thought I was different, and most people were auditory learners. So I find out, that when you start teaching someone, it is good to begin with saying "Our desire is to help you learn, and understand what we are teaching about Jesus Christ and His Gospel, how do you normally learn best?"
Right off the bat you know what you need to do, you know if you need to use hands on lessons (Tactile learning), or maybe you need to use object lessons or analogies (Visually learning), or if reading right out of the scriptures, and teaching them orally is best (Auditory learning).
It is VITAL that you teach them how they learn. Countless people on my mission remembered next to nothing of what they had been taught, because they were taught in a different manner than they learn.