A central idea in Christianity is that there is one God and he has revealed himself to us; he has shown us what he is like. He is revealed as Father, Son (Son as in the Son of the Father), and Holy Spirit. Many Christians begin and end most prayers by praying to God as he has been revealed to us. Hence they pray, “In the name of the Father, and of the Son, and of the Holy Spirit.” Not all Christians do this, however. They also tend to ask their prayers “through Christ our Lord” or “in Jesus’ name”.Also, Christians throughout history have understood that they are saved by Jesus, who died on a cross for our sins. They recollect this with numerous different cross symbols. The older gesture is to make a tiny cross on the forehead using the thumb to trace it. It has become combined as the larger gesture you recognize along with the prayer to God (I'm assuming this is the gesture to which you refer about shoulders and head. It is supposed to look like a vertical line down the body and a crossbeam across the body). So two specifically Christian truths are brought to mind by the gesture for them. Though the older gesture is still used all over the place, it doesn’t seem to appear in Hollywood movies, so you’ve probably never seen it. Also, depending on which Christian you ask, the position of the hand as the cross is traced has meaning, usually three fingers for the Trinity and two for how Christ is both God and man.Other postures and gestures are commonly combined with prayer. Kneeling, prostration of the body, raising hands in supplication, etc.