I've added a section "Prayer in Christianity: Biblical Rules and the Effectiveness of Prayer" to my page on prayer in monotheistic religions:
In Matthew 6:5-6 Jesus says you should pray in private so that others cannot see, behind closed doors, in secret and not in public. All four gospels describe Jesus as practicing what he preached: always praying in private or sometimes just amongst the disciples. Matthew also says that Jesus instructed that prayer not be repetitious, with 'much speaking' 'as the heathens do'. The criticism is made many times of those who bring attention to their own prayers: Prayer should be private. These instructions occur in no less than 18 places in the New Testament. Christians who make vocal their prayers for others are rebelling against God's wishes, as are those who prayer in repetitive chants or routines, something which is fairly common in Christian liturgy in most of Christendom.
There are many more versus about praying in the Bible. According to Jesus if Christians have just a tiny bit of faith, their prayers will come true, including such miracles as moving mountains and trees. For an example from each gospel, see Matthew 17:20, Mark 11:24-25 , Luke 17:6 and John 14:14. Critics do not have to do much work to find that these verses seem to be untrue. It shows us that the Bible is not a literal text full of literal truth. You could conclude that when the Bible says that prayers will come true that perhaps it means in the afterlife, in heaven, except that Jesus specifically states that he is talking about prayers physically coming true in the real world (e.g. regarding moving mountains and dunking fig trees). It is not therefore even full of symbolic truth; it appears in the cases of the 17 versus involved to actually be pure fantasy. It is easy to imagine how verses like these make believers very happy about the power of their religion, but I would hazard a guess that this source of happiness is rarely taken seriously by the believers themselves.
For complete lists of versus, see the full page: "Prayer in Christianity: Biblical Rules and the Effectiveness of Prayer"