The reason why many get discouraged in their ministry of Evangelism is that they don't the results as much as they expect. The results are usually measured by people by the number of converts. They think that the fruitfulness of a ministry should be assessed by the number of people who claim to have accepted Jesus as their Saviour through them.
What is the problem with this idea about measuring evangelism success? God rewards us based on our faithfulness to evangelize to as many people as we could and also live a life worthy of the message we bring. It is not based on the number of converts. Our way of measuring success is often flawed and different from God's.
It is God's job to change the heart of a person and open his or her spiritually blind eyes. It is our job to go and preach the gospel to people clearly.
The Salvation message must be clear. The core of the gospel is man's sin, Christ's atoning sacrifice, salvation through faith alone, by grace alone and that none of our good works can earn it or help keeping it. Unless we have child-like faith in Christ alone, we cannot enter heaven. Unless a person believes in the finished work of Jesus, he or she cannot be saved. Jesus paid it all.
Jesus asked us to "Go and preach the gospel", not to go and convert people, because that is the work of the Holy Spirit. We are just tools in the hands of God, to do His work. He works through us and we are rewarded for all our faithful deeds, which we do to please and glorify God.
Now, we should not only tell the gospel in words, but also live the gospel. We must act in a manner that is worthy of Jesus.