Rebbi taught, "Reflect upon three things and you will not come to sin: An Eye that sees, an Ear that hears, and all your actions are recorded in a Book."
(אבות פ"ב מ"א)
In Tanya, the Alter Rebbe (the first Grand Rabbi of Chabad) writes that before fulfilling a mitzva we should refresh our awareness of the Presence of HaShem (G-d) and His concern for our performance.
Whenever we do a mitzva, HaShem (so to speak) sets aside everything else in the physical world and the spiritual realms, and focuses his Sovereignty upon this one Yid who performs the mitzva, just as he was concerned with the service of Adam HaRishon who was the only human being on the planet. Realizing this will cause a person to serve HaShem with awe and fear, as if he were standing before a mortal king.
Although every Yid is born with this latent fear of HaShem, it can be a challenging task to uncover it so that it will be felt in one's day-to-day life. To this end a person must meditate upon HaShem's all-seeing Presence until he feels it.
(תניא ריש פמ"א ופמ"ב)
via Lma’an Yishme’u