Augustine’s most direct exposition on the Lord’s Prayer is found in his Letter 130, which is often called the Letter to Proba.
He wrote to the widow, Anicia Faltonia Proba in the year 412, in response to her request to Augustine about how she ought to pray. Augustine wrote to her that the Lord’s Prayer contains all the praise and petition that prayer requires. A person is free to express the same sentiments in other words if desired, but not to ask for anything that is either contrary to or beyond the scope of the Lord’s Prayer.
From what Augustine wrote in a number of his works (e.g., his Enchridion), his interpretation of the Lord’s Prayer is seen to coincide with his notion of pilgrimage. Each petition in the Lord’s Prayer is addressed to the Father, so as to remind a person of his or her position in relation to God and to neighbour. The familial term “Father” invites affection and fosters the desire to be a worthy son or daughter of the Father.
The term “our” cancels all thought of distinction between those who say this prayer. These persons are united in pilgrimage towards God, who should be the single focus of their spiritual desire. What a person seeks for self in praying, he or she should also seek for everyone else. According to Augustine, in the first three petitions of the Lord’s Prayer the pilgrim asks to be made acceptable to God through a process that begins in this life but which can only be completed in eternity.
In the final four petitions the pilgrim beseeches God for all that is necessary for the journey to eternity. All the petitions are directed to the goal of eternal life. Except for the request for forgiveness, each petition is an appeal for perseverance through the earthly pilgrimage to eternity with the Father Who is in heaven.
Augustine of Hippo: Letter 130 (written in the year 412). From Augustine to Proba. The full text. http://www.newadvent.org/fathers/1102130.htm and http://www.crossroadsinitiative.com/library_article/768/Letter_130_to_Proba_Augustine.html