1 Thessalonians 2:5 & 6a

"You know we never used flattery, nor did we put on a mask to cover up greed - God is our witness. We were not looking for praise from men, not from you or anyone else." NIV translation

Paul continues by explaining that they didn't come with tricks or having a hidden agenda and that God knows their hearts in this matter. He goes on to explain that they were not even seeking approval from men but only from God. An example of the impure motives comes from one of my true life experiences. 

As part of a "church" we would go out on Monday evenings calling on people that had visited the worship service. On one occasion, I went to do this with an elder of the "church" and he explained that he was doing it because we had to get more people in to help pay the bills. I never participated in the visiting ever again because that is an impure motive.

1 Thessalonians 2:6b & 7

"As apostles of Christ we could have been a burden to you, but we were gentle among you, like a mother caring for her little children." NIV translation

Paul continues to describe the method that God had them to use in bringing people to Christ. That method was simply speaking the truth in love and it is the same method that God would have us to use today. Many want to simply talk of love and even go so far as to compromise the truth while others simply try to beat down the struggling person and not show them love.