"But you, when you pray, go into your room and when you have shut your door, pray to your Father who is in the secret place; and your Father who sees in secret will reward you openly." [Matt 6:6]
I once heard a non-believer argue on the radio that Christians shouldn't hold public prayer meetings because "your Jesus" (as he put it) said not to pray in public. Was he right? If all you read is Matthew 6:6, then it certainly sounds that way, but as always, context is crucial.
In Matthew 6:1-18, Christ is warning against doing good things in public for the purpose of being seen by others. The Pharisees were notorious for doing this because they loved the praise of men. When they fasted, for example, they would walk around looking pitiful so that others would recognize that they were fasting and think they were doing something great.
This does not mean that you cannot do charitable deeds or pray in such a way that others might see you doing it, it just means that shouldn't be your motivation. Volunteer at the soup kitchen to feed others, not to be seen feeding others.
In fact, in the previous chapter, Christ said "You are the light of the world...Let your light so shine before men, that they may see your good works and glorify your Father in heaven." [Matt 5:14-16]. Reconciling these two passages it becomes clear that we are not to do good deeds for the purpose of being seen by others as the Pharisees did, but rather to glorify God. We are even encouraged to do them in such a way that others might see us so that God may be glorified, as long as it is God being glorified and not ourselves.