The Scriptures teach that we are saved by grace through faith. And Paul tells us that saving faith comes by hearing the word of God. And the content of that saving faith is that Jesus Christ is Lord, and that God has raised Him from the dead. Without such a belief and confession, no man can be saved, for such a belief and confession is the grounds of our salvation.
So above all, preach the cross of Jesus Christ. Preach Christ crucified. But as my pastor once pointed out: It's not the Jesus on the cross that can do anything for you, but it's the Jesus at the right hand of the Father in heaven that can do something for you. Unless men come to have a revelation that Jesus Christ is alive, they will remain dead in their sin.
So, to be a merely moralist in your preaching, and preaching the law will simply not do. While it is necessary to call out sin (as men must repent of their sins), all the conviction and repentance in the world will mean nothing unless they come to believe that Jesus Christ is alive from the grave.
If you closely study the book of Acts, you will find that was the true thrust of apostolic preaching. You can see that from Peter's preaching on the day of Pentecost to Paul's preaching before Felix & Agrippa. Peter preached "You Jews sinned by murdering Jesus Christ, whom God raised from the dead." Paul preached, "God's appointed a day when all men will be judged for their deeds, by One whom God raised from the dead!"
This in a nutshell is the gospel of Jesus Christ. The resurrection of Christ must not be merely an "oh, by the way..." thing we add to the end of our preaching. It must be the very center of it. And such preaching can only issue out of the life of a person who has indeed made Christ their very life, and are dependent upon the resurrection power of Christ. It's easy to fake true preaching by going out to do open-air with a bag of tricks you learned from Ray Comfort and WOTM. But what you can't fake is resurrection power.
The problem with so much street preaching is that some of us street preachers go out merely to convince men that they are awful sinners in the hands of an angry God. And while this is true, and such must be preached, don't take a truth and make it the truth. For such ought not to be the grounds of their repentance. For what turned the world upside down in the book of Acts was not convincing men they were awful sinners (the Pharisees had long been doing that)... but what turned the world upside down was that God raised Jesus Christ from the dead. As a result of such a belief, genuine repentance comes forth. For to believe such an awesome thing, that God raised a dead man back to life after three days, how can one ever remain the same after coming to believe such a thing?
Sadly though in my typing of this, many of you who read this will not hear. You'll go on with your silly moralizing sermons and bag of tricks. And why? Because you yourself have not truly had a personal revelation of the holiness of God and the resurrection of Jesus Christ. Instead, you will go out in your own power to preach, instead of going out in His. And as Art Katz has wonderfully shown, there is a difference between "went" and "sent."