According to Orthodox belief: "each man has a guardian angel who stands before the face of the Lord. This guardian angel is not only a friend and a protector, who preserves from evil and who sends good thought; the image of God is reflected in the creature - angels and men - in such a way that angels are celestial prototypes of men. Guardian angels are especially our spiritual kin. Scripture testified that the guardian ship and direction of the elements, of places, of peoples, of societies, are confided to the guardian angels of the cosmos, whose very substance adds something of harmony to the elements they watch over." (Sergei Bulgakov).
In one of the litanies in the Orthodox Divine Liturgy, there is an entreaty for: "an angel of peace, a faithful guide, a guardian of our souls and bodies," referring to our guardian angel. In most Orthodox prayer books intended for daily use there are specific prayers to our guardian angel.
Below is a longer prayer - a canon - to "the" (i.e. your) Guardian Angel. It can be said on any occasion as our guardian angel is always invisibly with us, but is one of the canons that can be privately said before taking holy communion.
