Fear of man. Fear of man or a desire for man's approval is what prohibits us from open air preaching. You might perceive it as you don't what to say, you don't what to do or where to go to preach but the fact is you are afraid of the response of the people who will both hear you and your friends and family who will know you are open air preaching.

What's the remedy?

First, do it anyway. 

The result of Adam and Eve responding to the Devil's temptation in the Garden to eat from the Tree of the Knowledge of Good and Evil where he said, "For God knows that in the day you eat from it your eyes will be opened, and you will be like God, knowing good and evil.” (Gen 3:5)

So in all aspects of life you will fear man not only in open air preaching.  Thus in all areas of life you just go ahead and do them despite the perspective of man. Obviously I'm speaking of righteous activities.

Second, realize as you begin open air preaching you will grow and learn how to preach and minister to people.  But the way to learn is by doing.  Study the Bible, pray and preach.  These are the three items to consistently engage in so you can increase in your knowledge and wisdom of open air preaching.

Last, you will find once you begin your 'natural' instincts will kick in; you will be surprised how much you know to do once you begin.

This is a general overview of the obstacle.  More details to follow.  If you have specific questions please send them.