What is the significance of the sign of the Cross? Why is it performed when it is? When is it preformed besides after a prayer? Fr. John Whiteford answers these questions on his blog.
Making the Sign of the Cross is perhaps the most common act of piety Christians engage in throughout the day, and this is a practice of Apostolic origin. We do it to remind ourselves of the Cross that was the means of our salvation, and the Savior who was crucified upon it. It is the first thing we do at the beginning of the day, and the last thing we do when we lay down to sleep. we do it before we begin any task, and when we conclude it. We make the Sign of the Cross when we are in danger or tempted, and in thanksgiving. By making the Sign of the Cross with reverence, we are strengthen by the power of the Cross, and we confess our Faith in Christ to the world.
On the feasts of the Cross we sing the hymn "The Cross is the guardian of the whole world! The Cross is the beauty of the Church! The Cross is the strength of kings! The Cross is the support of the faithful! The Cross is the glory of the angels and the wounder of the demons!"
St. Athanasius the Great, for example wrote: "demons used to deceive men's minds by taking up their abode in springs or rivers or trees or stones and imposing upon simple people by their frauds. But now, since the Divine appearing of the Word, all this fantasy has ceased, for by the sign of the cross, if a man will but use it, he drives out their deceits" (On the Incarnation of the Word, 47:2).
St. Cyril of Jerusalem said: "Let us not then be ashamed to confess the Crucified. Be the Cross our seal made with boldness by our fingers on our brow, and on everything; over the bread we eat, and the cups we drink; in our comings in, and goings out; before our sleep, when we lie down and when we rise up; when we are in the way, and when we are still. Great is that preservative; it is without price, for the sake of the poor; without toil, for the sick; since also its grace is from God. It is the Sign of the faithful, and the dread of devils: for He triumphed over them in it, having made a shew of them openly; for when they see the Cross they are reminded of the Crucified; they are afraid of Him, who bruised the heads of the dragon. Despise not the Seal, because of the freeness of the gift; but for this the rather honour thy Benefactor (Catechetical Lecture 13:36).
Much more could be said, but others have already covered very well.
To begin with, I would recommend you read what Fr. David F. Abramtsov wrote on this subject in his classic catechetical book "The Orthodox Companion," which is available here:
"The Sign Of The Cross," By Rev. David Abramsov
For more details on when the Sign of the Cross is made, this is covered extensively in the Jordanville Prayer Book, in an appendix entitled "How One Should Pray in Church." You can purchase a copy by clicking here (and it is an excellent prayer book to have anyway). But a good way to learn when to make the sign of the Cross during the services is to pay attention to what everyone else is doing during the services, and emulate what you see, especially with regard to those who you know are regular and pious members.
You can also read some more quotes of the Saints and Fathers of the Church here:
Stump the Priest: Wearing a Cross
Stump the Priest: Making the Sign of the Cross
Stump the Priest: The Veneration of the Cross