Oratio ad angelum custodem
Credo quod sis angelus sanctus, a Deo omnipotente ad custodiam mei deputatus. Propterea peto, et per illum qui te ad hoc ordinavit, humiliter imploro, ut me miseram fragilem atque indignam semper et ubique in hac vita custodias, protegas a malis omnibus atque defendas, et cum Deus hinc animam meam migrare iusserit, nullam in eam potestatem daemonibus habere permittas, sed tu eam leniter a corpore suscipias, et in sinu Habrae suaviter usque perducas iubente ac iuvante creatore ac salvatore Deo nostro, qui est benedictus in saecula saeculorum. Amen.
Prayer to the Guardian Angel
I believe that you are the holy angel, appointed to watch over me by almight God. So I beg you, through him who ordained you to this duty, and I implore you humbly, that you will always and everywhere guard me, wretched, weak, and unworthy, in this life, and when God commands my soul to leave this world, do not permit the devil to have power over it, but rather softly take it up from my body and into the bosom of Abraham gently lead it, with the bidding and aid of our savior and creator God who is blessed for the age of ages (ie, for eternity). Amen.
Reprinted in Andre Wilmart, Auteurs spirituel et textes devotes du moyen age latin (Paris, 1971, repr. of 1932 ed.)
A photo of a fully-armored St. Michael statue in Salisbury that I took in summer of 2010 (not public domain - image is my property and you may not reuse it without my permission).