When we pray for someone to come to Christ, we can be assured that we are praying according to the will of God. As 1 Peter 3:9 tells us, "[God] does not want anyone to be destroyed, but wants everyone to repent."
The apostle Paul specifically asked for prayer as he preached the gospel. He wrote to the believers in Ephesus:
Pray in the Spirit at all times and on every occasion. Stay alert and be persistent in your prayers for all believers everywhere. And pray for me, too. Ask God to give me the right words so I can boldly explain God's mysterious plan that the Good News is for Jews and Gentiles alike. (Ephesians 6:18–19)
People will sometimes ask how they can pray for me as a crusade is approaching. My response is the same as Paul's. I take what I do very seriously. And even though I write my messages out word for word, there are times when I get up to speak and some thought will come to me that I had not considered previously. It is not in my written notes, but I will share it anyway. Then people will come to me later and say, "It is when you said that one thing that I decided I wanted to become a Christian."
Think back on the day that you became a believer. If it was through preaching, you will probably remember one specific thing that was said. You probably don't remember point number four or five or that you really enjoyed certain verses. You just heard that one thing, and something took hold in your heart. So I pray for those right words as I get up to speak. And we all need to pray for those right words from God.