From a netfriend:

There's a difference between repetition and "vain" repetition. Basically we greet people in an identical way every day, yet that repetition is not meaningless. It is a greeting. If we were to say "Hi! Have a good night?" five, six or twenty times a day it would be meaningless, but it is of value first thing in the morning. The same thing applies in many little ways. We may ask someone several times during the day if they'd like some tea, because it is meaningful several times a day - at least in tea drinking households. But I have a friend who never drinks tea. For me to ask him if he wants tea would be to insult the information he has already given me. I think prayer, which is just a conversation with God, is like that. We make it meaningful by our personalities, our communication styles, our "being" with the Lord. For us to respond in another manner is for us to pray in "vain".