St. Francis de Sales proposes the practice of the presence of God as essential for building a spiritual life. We must always begin our prayer by making ourselves aware of him to whom we are speaking. He is always present; we just forget.
Over and over again.
And St. Francis uses four different means to bring the reality of God's presence home.
To bring home to our minds that God is everywhere and every place and every time. He uses the image of a bird - no matter where it goes, it encounters the air. It is upheld by it; moves through it with ease and joy; not able to survive if the air went away. Likewise, in all things we live and move in the presence of God.
C.S. Lewis, in his book, Perelandra, uses the same idea with a different image. He talks about the air being "too crowded to breathe" when one was of an independent mind. In the act of surrender, it was rather "a sort of splendor of eatable, drinkable, breathable gold." In this medium, we live and walk and act.
The second way of placing yourself in this holy presence is to think that not only is God in the place where you are, but that he is in a very special manner in your heart and in the depth of your spirit...he is the heart of your heart.
To focus on the depth of our hearts, knowing that through Baptism we are particularly united to him. He lives within us.
The third way is to consider our Savior, who in his humanity looks from Heaven upon all persons in the world, but particularly upon Christians who are his children, and especially upon those who are in prayer, whose actions and behavior he observes.
In some ways, this is the easiest of the ways. We can picture Jesus - however we picture him - and know him more easily as a human person. His Incarnation makes possible the image we have. He, though God, is a man, and, in his manhood, is approachable by our minds and affections. In this method, we see him looking at us as we begin to pray and can bring our hearts more easily to the reverence, confidence and love we need to approach him.
The fourth way consists in making use of the imagination alone, representing to ourselves the Savior in his sacred humanity, as though he were near to us...but if the most holy sacrament of the altar be present, then this presence will be real and not merely imaginary.
In all of the methods above, we worked from reason and reality. The thoughts we used were all simply true. In the fourth way, he recommends that we "make a story," and picture the Lord speaking to us, sitting near us, spending time with us. This can be very helpful if you have a good imagination. He brings up the very real point, though, that if we are in the Real Presence of God, our imagination is really only telling us things that are actual. He is present. He is with us. He does look directly at us and speak plainly. Our imagination, at that point, is only giving reality the barest structure.
All of these means are to be used as helps - and never more than one at a time. The point of the practice of the presence of God is not to "do it right," but to bring our hearts to reverence, to praise, to quiet, to focus, so that we can pray rightly to the God who loves us dearly dearly.