"We loved you so much that we were delighted to share with you not only the gospel of God but our lives as well, because you had become so dear to us." NIV translation
Another part of the method behind the spread of the gospel in Thessalonica was that Paul and his team shared their lives with the people there. They lived among the common folks (if even for only a few weeks) and let the people there see their faith in action. Many today want to proclaim the Word but do not want to lower themselves to be where those that need it are located. Many want the people to clean themselves up and then come to their building to receive the Word. That is not what Jesus (or Paul) did as both of them went to those that needed to hear the Word and stayed among them (if even for only a short time).
"Surely you remember, brothers, our toil and hardship; we worked night and day in order not to be a burden to anyone while we preached the gospel of God to you." NIV translation
We see here that Paul and his team did not count on the people they were trying to reach or anyone else to supply their needs. They worked to support themselves and also shared the gospel. We have gotten so far from this model today as so many people are "paid staff" at local "churches" and it has become more of a job than focused on proclaiming the gospel.
Then, many of the people in these "churches" believe that they are no longer responsible for sharing the gospel as they aren't the ones that are on the "paid staff".
"You are witnesses, and so is God, of how holy, righteous and blameless we were among you who believed." NIV translation
Paul is telling us here that the method that they used was simple in that they practiced what they preached. He says here that the Thessalonians saw that they were different and set apart for God which inspired them to do so as well. One of the main problems today is that many who say they are called to spread the Word around the world do not walk the talk.
"For you know that we dealt with each of you as a father deals with his own children, encouraging, comforting, and urging you to live lives worthy of God, who calls you into his kingdom and glory." NIV translation
Paul reminds the Thessalonians that he had treated them as children. The first duty of any father is to train the children in the ways of the Lord.