John Chryssavgis, whose first-rate book on the Desert Fathers and Mothers is the subject of an earlier post on this site, has some insights on the relationship between ascesis (a systematic set of spiritual practices) and theosis (the process of gradually achieving union with God). They are found in "The Spiritual Way", a chapter in The Cambridge Companion to Orthodox Christian Theology, eds Mary B Cunningham and Elizabeth Theokritoff, Cambridge University Press, 2008, pp 160-162.

The ascetic way, then, is a way of authentic liberation and communion. For the ascetic is the person who is free, uncontrolled by attitudes that abuse the world; uncompelled by ways that use the world; characterised by self-control, by self-restraint, and by the ability to say 'no' or 'enough'. Indeed, asceticism aims at refinement, not detachement or destruction. Its goal is moderation, not repression. Its content is positive, not negative: it looks to service, not selfishness; to reconciliation, not renunciation or escape: 'Without asceticism, none of us is authentically human' [Kallistos Ware].

Unfortunately, however, centuries of misunderstanding and abuse have tainted the concept of asceticism, identifying it either with individualism and escapism or else with idealism and angelism. Both tendencies verge on the point of dis-incarnation, promulgating enmity towards the world. Yet, at least in its more authentic expression, asceticism is a way of intimacy and tenderness, a way of integrating body, soul and society. In this respect, asceticism is essentially a social discipline. Moreover, it is never practised in a way that would insult the Creator. It is no wonder, therefore, that even after years of harsh and frugal living, the early desert Fathers and Mothers would emerge in their relationships as charming and compassionate, accessible and tranquil...

In the final analysis, the aim of asceticism is to regain a sense of wonder, to be filled with a sense of goodness and of God-liness. It is to see all things in God and God in all things. And it is precisely here that ascesis encounters theosis. for the most divine experience is to discover the wonder of God in the beauty of the world and to discern the limitless nature of grace in the limitations of the human body and the natural creation. There are those among us who may well be converted 'suddenly with a light flashing from heaven'(Acts 9:3) or be 'caught up to the third heaven'(2 Cor 12:2). Yet such ecstasy is experienced by very few--'scarcely one among ten thousand...indeed, scarcely one in every generation', according to Abba Isaac the Syrian. It is no wonder, then, that the desert Fathers encourage their disciples to restrain someone rising to spiritual heights: 'The old men used to say: "If you see someone climbing toward heaven by his own will, grab his foot and pull him down; for this will be for his own good"'. The ascetic literature clearly demonstrates a preference for the more lowly experience of those who have known their passions and recognized their failures. John Climacus refers to them as 'blessed': 'I saw...and was amazed; and I consider those fallen mourners more blessed than those who have not fallen and are not mourning". While the end of ascesis may be the vision of God or theosis, the way of ascesis is none other than the daily life of self-knowledge or integrity, carved out of the ordinary experience of everyday life perceived in the extraordinary light of the eternal kingdom. It is the gradual--and, as a result of our resistance, painful--process of learning to be who you areand do what you do with all the intensity of life and love. 'An old man was asked: "What is it necessary to do to be saved?" He was making rope; and without looking up from the work, he replied: "You are looking at it"'. In this way, the ascetic way defines in a uniquely tangible and concrete manner the theological doctrines concerning the original creation of the world, the divine Incarnation of the Word, and the age to come that we expect.