We practice agency in every choice that we make. Not every choice is a big one, but we have the ability to choose, resist, embrace, or act upon any external influence, and that in itself is amazing.
It doesn't just apply to Mormonism; its an all-encompassing principle that touches everyone. In the tests and trials of life, agency is the most valuable measuring stick with which we can be judged. If you think about it, without the ability to choose for oneself, a test would be meaningless. We would would all pass with flying colors, but none of us would grow. We would remain stagnant beings, choosing right because that is what we were forced to do, not because we actually were righteous. It would be like taking a child through school and giving them all the answers along the way, all the way through med-school and then asking them to perform brain surgery. What was the point in testing if you knew the outcome?
It is therefore of the utmost importance that we are able to choose for ourselves how we will act. An important thing to remember is that, although we are free to choose our actions, we are not free to choose the consequences. It is within our capability to choose evil, but we do not get to choose what happens to us because of that evil. Others can use their agency, and their choices more often than not, effect us, and our's them. Such is the way of this test we call life. Even if some outside force, be it the choice of another, or pure chance, effects us for the worse, we still have the choice of how we will move forward. We can react, build, adapt, create, improve, and much more. It ultimately boils down to choice between two masters and two paths; God, Jesus Christ, and all that is good, or Satan and all that is evil. This is our choice, this is everyone's choice. And I feel privileged to be able to make it.
For a similar article on agency, read this post.