Holy Communion: Our Life in Christ:
The Sacrament of Holy Communion is the third of the Sacraments of Initiation. Even though we are required to receive Communion at least once per year (our Easter Duty), and the Church urges us to receive Communion frequently (even daily, if possible), it is called a sacrament of initiation because, like Baptism and Confirmation, it brings us into the fullness of our life in Christ.
In Holy Communion, we are eating the True Body and Blood of Jesus Christ, without which "you shall not have life in you" (John 6:53).
Preparing for the Sacrament of Holy Communion:
Because of the intimate connection of the Sacrament of Holy Communion to our life in Christ, we must be free of any grave or mortal sin before receiving it, as St. Paul explained in 1 Corinthians 11:27-29. Otherwise, as he warns, we receive the sacrament unworthily, and we "eateth and drinketh damnation" to ourselves.
If we are aware of having committed a mortal sin, we must participate in the Sacrament of Confession first. The Church sees the two sacraments as connected, and urges us, when we can, to join frequent Confession with frequent Communion.
Making a Spiritual Communion:
If we cannot receive Holy Communion physically, either because we cannot make it to Mass or because we need to go to Confession first, we can pray an Act of Spiritual Communion, in which we express our desire to be united with Christ and ask Him to come into our soul. A spiritual communion is not sacramental, but prayed devoutly, it can be a source of grace that can strengthen us until we can receive the Sacrament of Holy Communion once again.
The Effects of the Sacrament of Holy Communion:
Receiving Holy Communion worthily brings us graces that affect us both spiritually and physically. Spiritually, our souls become more united to Christ, both through the graces we receive and through the change in our actions that those graces effect. Frequent Communion increases our love for God and for our neighbor, which expresses itself in action, which makes us more like Christ.
Physically, frequent Communion relieves us of our passions. Priests and other spiritual directors who counsel those who are struggling with passions, especially sexual sins, often urge frequent reception not only of the Sacrament of Confession but of the Sacrament of Holy Communion.
By receiving Christ's Body and Blood, our own bodies are sanctified, and we grow in our likeness to Christ.
In fact, as Fr. John Hardon points out in his Modern Catholic Dictionary, the Church teaches that "A final effect of Communion is to remove the personal guilt of venial sins, and the temporal [earthly and purgatorial] punishment due to forgiven sins, whether venial or mortal."