The reason why many get discouraged in their ministry of Evangelism is that they don't see the results as good as they expect. The results are usually measured by the number of converts or new church goers. They think that the fruitfulness of a ministry should only be assessed by the number of people who claim to have accepted Jesus as their Saviour through them.
There is a problem with this way of 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. Only God can change people, not our human wisom or effort. 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 conversion 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.