Recently a teen from my youth group asked me if I could write down my testimony for him (I found out later he was collecting them to later make into a song). As I was writing, I found I had to keep catching myself in order to not use the phrase "ask Jesus into my heart." I know many Christians have used this phrase before with good intentions. But I have learned recently that it is misleading and simply not Biblical. For a more lengthy treatment on the topic, see the following article: http://www.wretchedradio.com/pdf/ten_reasons.pdf
Ask yourself, how does someone become a Christian? I hope your answer would say something about repentance and putting one's faith & trust in Jesus Christ (see Acts 2:38 for example). "Repentance" & "faith" to me seem fairly straight forward, but I'll say what I mean when I use those terms so there is no confusion. When I say "repentance" I mean - admitting that you have sinned against an infinitely holy God, truly being sorry, and turning from your sin (it is the "turning" part that is true repentance). When I say "faith" (faith in Jesus in particular) I mean - believing that Jesus is who he says he is (the second person of the triune Godhead, God incarnate, who came down in human flesh to live a perfect life, to die on a cross to save mankind from sin, and to rise from the dead defeating sin & death). "Trust" can be a little trickier to explain. It's carries a bit of a stronger connotation than just faith or belief. Even the demons believe there is one God (James 2:19), but you trust in Jesus -- it's trusting a person. It means you trust everything he says and his way is the best way. You entrust your life to him; that is what becoming a Christian means.
I know many Christians and churches have used the phrase "ask Jesus into my heart" with good intentions. But I fear it can really do more harm than good by leaving out the key components of repentance and trust.