Here is the famous Latin prayer to one's guardian angel:
Angele Dei,qui custos es mei,me, tibi commissum pietate superna,illumina, custodi,rege et guberna.Amen.
The traditional English translation:
Angel of God, my guardian dearTo whom God's love commits me here,Ever this day, be at my side,To light, to guard, to rule and guideAmen.
An Eastern Orthodox prayer to the Guardian Angel:
O Angel of Christ, my holy Guardian and Protector of my soul and body, forgive me all my sins of today. Deliver me from all the wiles of the enemy, that I may not anger my God by any sin. Pray for me, a sinful and unworthy servant, that thou mayest present me worthy of the kindness and mercy of the All-holy Trinity and the Mother of my Lord Jesus Christ, and of all the Saints. Amen.(Elsewhere: Fountain of Elias, The Pious Spinster)