"In all undertakings -- when we enter a place or leave it; before we dress; before we bathe; when we take our meals; when we light the lamps in the evening; before we retire at night; when we sit down to read; before each task -- we trace the sign of the cross on our foreheads."
Saint Augustine also mentions the common practice as normative in the life of the church. Even the Reformers in the 16th century maintained the practice as a form of worship. Crossing yourself is a physical expression of worship and a symbol of the person and nature of Jesus Christ. The two fingers and thumb touching one another represent the Holy Trinity, Father, Son, Holy Spirit. The two fingers touching the hand represent the two natures of Christ, fully God, fully Man.
The oldest and Eastern Orthodox form of crossing oneself is to touch the two fingers and thumb to the forehead, then to the breast, then to the right shoulder, then to the left shoulder. One may say, "In the name of the Father and of the Son and of the Holy Ghost. Amen."