Christians do not prostrate on Sunday because this is the day we always celebrate the Resurrection, and we recall how God is able to make us stand. Our standing reminds us of the resurrection. There are one or two Sundays a year when we do prostrate (the Third Sunday of Great Lent (Veneration of the Cross), and the Exaltation of the Holy Cross, if it falls on a Sunday. On this day, the hymn "Before Thy cross", which we sing while prostration, teaches us what our prostrations mean:
Before Thy Cross, we fall down in worship, O Master, and Thy holy Resurrection we glorify.
When we prostrate, it is in worship of the Risen Lord, and when we raise ourselves up, we recall the resurrection.
A Christian prostrates when he makes the sign of the cross, and falls to his hands and knees (it is usually easier to have the hands touch the floor a moment before the knees), and bows his head to the ground, then gets back up