How Should We Receive the Blessed Sacrament?

Before we receive Christ, we must examine our conscience and ask ourselves if we are free of mortal sin. If we have mortal sin on our soul, then we cannot receive Communion, for it is a sin to receive Him in that state. We can receive Him only in the state of grace. Venial sin does not prevent us from receiving the Eucharist, but it does prevent us from receiving more of the graces that we receive from Communion.
St. Paul says in 1 Cor. 11:27-28 that we should not receive in an unworthy way:
Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup.
As St. Paul says, to receive the bread of our Lord in the state of mortal sin defiles the Eucharist. 
When we walk up to receive the Eucharist, remember Whom we are receiving. We are truly taking in the Body of Christ, the man who suffered and died for the sake of our sins! Meditate on the beauty of the Eucharist: Christ gave His Body and Blood to us, and now we get to receive Him! So how should we receive the Lord of Lords? Should we get down on our knees or stand? Take Him by tongue or by hand?
Think of it this way: if you were to see Jesus, how would you react? Surely, you would show Him the most respect you can possibly express. Would you fall to your knees in awe and wonder like the Magi and the disciples, or would you just shrug your shoulders and treat Him as if He were dust? Most likely, you would fall to your knees and worship Him. Thus, when we approach the priest or deacon to receive Jesus, we want to be as devout as possible, because Christ is present before us.
Receiving the Eucharist on our knees shows that we are physically offering our submission to God by humbling ourselves. When we kneel, we demonstrate that we are ready to unite ourselves to the suffering and death of Christ with the hope that we will be redeemed.
Bishop Athanasius Schneider comments on why we should kneel for the Eucharist: “[kneeling] symbolizes the attitude of humility and the spirit of spiritual infancy, which Jesus himself requires from all who want to receive the kingdom of God.”
Although receiving in the hand is acceptable, it is not as reverent as receiving the Eucharist on the tongue. In fact, we should not be touching the Eucharist at all unless there are circumstances that make it absolutely permissible. The host is extremely sacred, for it is Christ’s Body, Blood, Soul, and Divinity. Bishops, priests, and deacons are the only ones who should be allowed to touch this Blessed Sacrament. The bishop and priest, by virtue of the sacrament of holy orders, are acting “in persona Christi,” which means “in the person of Christ.” The deacon also represents Christ as a servant by ministering to the parish and assisting at Mass.
Saint Thomas Aquinas explains the sacredness of the Eucharist and how it must be properly handled:
[O]ut of reverence towards this Sacrament, nothing touches it, but what is consecrated; hence the corporal and the chalice are consecrated, and likewise the priest’s hands, for touching this Sacrament. Hence, it is not lawful for anyone else to touch it except from necessity, for instance, if it were to fall upon the ground, or else in some other case of urgency. (Summa Theologiae, III, 82, 3)
If we receive with our hands, we also face the possibility that particles of the Blessed Sacrament will remain on our hands, which may be washed away or even fall to the floor.
The Lord deserves to be united directly to us through the mouth, rather than in our hands first. 
Going Forth with the Effects of the Eucharist
Once the sacred host is placed on our tongues, we should not let it dissolve in our mouths, nor swish it around as if it were a mint. As we kneel before Christ, we should spend time praying and meditate on Christ’s sacrifice and the beauty of the Eucharist. We can spend that time thanking the Lord, asking Him for blessings or help in being able to love and obey Him more, or simply adoring Him. There are multiple prayers after Communion that we can pray: the Anima Christi, the Prayer of St. Thomas Aquinas after Communion, the Prayer of St. Bonaventure, the Offering and Prayer of St. Ignatius Loyola, etc.
After we take in the Body of Jesus, we carry the effects given to us by this sacrament. Ven. Fulton Sheen in his Sunday Missal explains the effects of the Eucharist:
When I rise from the communion rail I not only have the divine Life in my body, but I am also about to carry with me into the world something of his Death, dying to everything that would separate me from him, dying to my concupiscence, my pride, my lust and my anger in order that there might be nothing between the Lover and the loved.
As Ven. Fulton Sheen wrote in his Sunday Missal, we carry into the world Christ’s sacrifice for us. By receiving our Lord, we become in union with Christ by love – not physically, but spiritually. This union increases our sanctifying grace and assists us to grow in our love for God, as well as our neighbor. Holy Communion helps us persevere against mortal sins and venial sins, lessening our inclinations to sin and guiding us to perform good works.