O Ruler of all, Word of the Father, O Jesus Christ, Thou Who are perfect: for the sake of the plenitude of Thy mercy, never depart from me, but always remain in me, Thy servant. O Jesus, Good Shepherd of Thy sheep, deliver me not over to the sedition of the serpent, and leave me not to the will of Satan, for the seed of corruption is in me. But do Thou, O Lord, Worshipful God, holy King, Jesus Christ, as I sleep, guard me by the Unwaning Light, Thy Holy Spirit, by Whom Thou didst sanctify Thy disciples. O Lord, grant me, Thine unworthy servant, Thy salvation upon my bed. Enlighten my mind with the light of understanding of Thy Holy Gospel; my soul with the love of Thy Cross; my heart with the purity of Thy word; my body, with Thy passionless Passion. Keep my thought in Thy humility, and raise me up at the proper time for Thy glorification. For most glorified art Thou, together with Thine unoriginate Father, and the Most Holy Spirit, unto the ages. Amen.

Or briefly:
Guard me, O Lord, by the power of Thy holy and life-giving Cross, and keep me from all evil.