Personally I think it's best to allow a person to repent when they are good and ready, rather than rush them into making a decision on the spur of the moment and then having them either backslide and turn away from God, or worse still have them remain in a situation where they think they are a Christian but they have never really repented. God can still use the sinner's prayer, but it needs to come from the heart of the person in a natural way as a result of the working of the holy spirit in that persons life.
Also related to this topic is that of follow up. While some follow up of Christians and discipleship is biblical, if we have to be exerting a huge amount of effort to get the new Christian to read their Bible and come regularly to church then it could be that they aren't really saved and as Ray Comfort said they could just be "a still-born baby" - i.e. a false convert.
In my many discussions with Atheists I've found that many of them say that they were once Christians who said the sinner's prayer and went to church for a while, but eventually they turned away from God. In those situations it's possible that they were saved, but it's also possible (and quite likely?) that they were never really a Christian at all.
1 John 2:19 says, “They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.”
In relation to those who say that they were once Christian but now they're unbelievers Sye Ten Bruggencate says "Your reasoning was never submitted to Christ – if it was you wouldn’t have been able to reason your way out of Christianity.What was the foundation of your reasoning then? What is it now?"
If the foundation of your reasoning in the past was not the Word of God then it is likely that you never were a true Christian.