Of course the definitive work on Practicing the Presence of God is by Brother Lawrence, but I found this passage by Traherne in “Happiness and Holiness.”
The least Slip of our Heart (out of the Devine presence) is Like the fall of Angels, So is the Least Cessation wherin we forget to make God our Supreme End. I ought therefore evermore in the Beginning of every Enterprize; to remember God, and aime at His Glory as my Supreme End. When I forget Him I walk in Darkness, when I aim at my Self it is in vain Glory. For He Lifteth up Himselfe above God that maketh Himself his Last End. In Ever Conference therfore Discours and enterprize I must actually remember His presence, and Direct my Intentions to His Glory. Not to do it, not to do it is the beginning of Error and of all Calamity. For by forbearing to do this I wander in the Dark, and become Subject to all Transgression, for from the very first moment wherein a man doth otherwise He is alienated from God, made weak, Apt to be Led into all Temptations.
Select Meditations, III, 75
Note the insistence upon closeness and immediacy as the primary characteristics of the relationship with God. This relationship is both intimate, i.e. concerned with exchange of persons, and immanent, i.e. always present and always at least implicitly desired and chosen.
Traherne and Brother Lawrence go further, though, in making the awareness of God’s immanence a matter of spiritual discipline. For Brother Lawrence it was the heart and soul of his spiritual life, the simple awareness of God’s presence in every moment of ever day, in every detail of every task. This awareness was not haphazard or subject to changing environment and circumstances. It was disciplined and focused and practiced. Brother Lawrence practiced this awareness until it became his default way of viewing the universe regardless of his external circumstances.
There is something very powerful about this devotion. It has gradually been working its way into the center of my spiritual life as well, as I think eventually it must if our spiritual life is going to move from activity to identity. At any rate, at some point the prayers we say at set prayer times must expand their borders, so to speak, and bleed over into the rest of the day. They must take over our lives, so everything else, every other part of our conduct, flows from them and is informed by them. The easiest and simplest way I have found of doing this (novice that I am) is to practice this awareness of God’s presence.