Always keep in mind that you will never be free from fear, especially just before you get up to open-air preach. Overcome it through thoughts of the fate of the ungodly, the sacrifice of the cross, and the fact that God is watching you. I have known men who said that it was less fearful for them to skydive for the first time than to open-air preach. So think of the worst-case scenario if something goes wrong with both. In skydiving, if the parachute fails to open or becomes twisted, you fall to an unspeakably terrifying death. In open-air preaching, you may make a fool of yourself and dent your ego. There is no comparison. So just do it, and God will be with you.
Haggai 2:5 According to the word that I covenanted with you when you came out of Egypt, so My Spirit remains among you; do not fear!