The Catholic Church encourages the faithful to make frequent, even daily, Communion. Today, the normal opportunity to receive the Eucharist comes at daily Mass. (In the past, many parishes, especially in cities, distributed the Eucharist before and after Mass to those who weren't able to attend the whole Mass.)
When we cannot make it to daily Mass, however, we can still make an Act of Spiritual Communion. This involves expressing our faith in Christ and in His Presence in the Eucharist, and asking Him to unite Himself with us. The basic elements of an Act of Spiritual Communion are an Act of Faith, an Act of Love, a desire to receive Christ, and an invitation to Him to come into your heart.
The following texts present one modern and one traditional translation of a popular form of an Act of Spiritual Communion written by St. Alphonsus de Liguori. You can memorize either version or use one as a guide to offer your own Act of Spiritual Communion.
My Jesus, I believe that You are present in the Most Holy Sacrament.
I love You above all things, and I desire to receive You into my soul.
Since I cannot at this moment receive You sacramentally, come at least spiritually into my heart. I embrace You as if You were already there and unite myself wholly to You. Never permit me to be separated from You. Amen.
My Jesus, I believe that Thou art present in the Blessed Sacrament.
I love Thee above all things, and I desire Thee in my soul.
Since I cannot now receive Thee sacramentally, come at least spiritually into my heart. As though Thou wert already there, I embrace Thee and unite myself wholly to Thee; permit not that I should ever be separated from Thee.
The most common occasion for making an Act of Spiritual Communion is when we cannot fulfill our obligation to attend Mass on a Sunday or Holy Day of Obligation, whether because of illness, bad weather, or another reason outside of our control. Many also make an Act of Spiritual Communion when they can and do attend Mass but something prevents them from receiving sacramental Communion that day, such as a mortal sin that they have not had the opportunity to confess yet.
But our Acts of Spiritual Communion do not need to be confined to those times. In an ideal world, it would be best to attend Mass and receive Communion every day, but we can't always do so. We can, however, always take 30 seconds or so to make an Act of Spiritual Communion. We can even do so multiple times a day—even on days when we have been able to receive the Eucharist. Why do that? Because each Act of Spiritual Communion that we make increases our desire to receive sacramental Communion and helps us avoid the sins that would make us unable to receive Communion worthily.