The goal of preaching is for your audience to take action.
Thus to determine what you want to say to your audience you must first determine what you want them to do once they hear your message.
For example as evangelists the typical goal for your audience is for them to repent and believe in Jesus Christ. Thus your message would lead you to calling your hearers to repent and believe.
But perhaps you are more concerned about the lack of sanctification of the Christians in your audience. Or that their faith rests not upon Christ alone but in something they have done. Or maybe they have errant theological views or are listening to false teachers, etc.
Whatever your goal may be for your audience examine yourself for what the goal is prior to preaching to them. As simple as this may seem if you do not know what you want your audience to do then your message will likely be unclear and you will probably not be motivated to routinely go and endure the challenges of open air preaching so your audience can hear your message.
But if you are aflame with a goal for your audience then you will take the necessary steps in your life for them to hear your message so they can respond.
Once you determine what you want your audience to do then you can develop your message. Developing your message is not exactly a call to sermon preparation but it is a call to determining how you will develop your message.
For instance what texts or theological themes emphasize your message. Which books of the Bible are a foundation for your message. Are there any contemporary preachers who appear to be expert in preaching the message you want to preach. Do your homework to develop a solid foundation from which you can draw for your messages.
In conclusion, you as a preacher want your audience to do something. Your first job, before you consider open air preaching techniques, is to determine what you want your audience to do and then develop a means of crafting messages to lead them to that response.
[This is a series of messages on Open Air Preaching. This is #1.]