This is something about teaching that took me a long time to really grasp, so I wanted to share some of my experience with the hope that it will help someone else.
I have taught Primary and Cub Scouts several times in several different wards, but there has been a big difference in my teaching in our current ward to all of those other times. There are probably a few reasons for this, but I think a big one is that I have been teaching with a purpose.
The difference seems subtle, but the effect is big.
The first year teaching in this ward, I decided that I wanted to get the students excited about the scriptures. It was not hard, because I love the scriptures, especially the Book of Mormon, which was what we were studying that year. The Book of Mormon is full of so many great stories that eight-year-olds can enjoy, especially when told by someone who is enthusiastic about the stories. The students were excited to come back every week to hear the next story. (Not every week was perfect, of course, but I think most everyone enjoyed being there most of the time. Then again, my memory may wear rose-colored glasses.)
Last year, teaching the same group of boys again, this time as ten-year-olds, I received an impression that I needed to be preparing them to go on missions. Ever since, we have kept that goal in mind when we plan and teach our lessons. We also try to keep the purpose stated at the beginning of the lesson in mind as we teach, but if there is some way we can relate it to missionary work or some skill that is valuable to a missionary (such as knowing the scriptures or gospel better or listening to the Spirit), that becomes the main purpose of the lesson. If we end up on a tangent that is mission-related, we go with it.
We are teaching the same lessons we would teach otherwise out of the same manual, but I can see a difference in the boys. You will have to wait another seven years, though, before I can really tell you how things turn out.
In Scouting, we have our goals spelled out for us with the Aims of Scouting and the Purposes of Cub Scouting. If you try to keep those goals in mind when planning, you will see a difference. You may think the difference in what you teach is subtle, but over time there will be a big difference in the effect.