Now I am pretty sure there is no manual on how to prep for speaking at a camp. I am not even sure if what I do is the best but here is how I have learned to best prep for speaking at a camp.
Study the Outline
Normally when you speak at a camp you will be given an outline, or some sort of document, that will help you understand the theme of the camp. Study this! If the camp wants you to preach on X,Y and Z then you better be familiar with X,Y and Z. From my experience the camp will give you some direction on where they are heading and the theme they have. The nice thing is the directions normally mean I get to use the freeway (I have lots of room to work) and normally don't have to take the country room (less room to move in).
If the camp gives you the material 6 weeks out; start studying. If the camp gives you the material 6 months out; start studying. I have found that the longer I can just think about the text, direction and theme the better I am. I am not a natural thinker so I need more time than your average person. When I get an opportunity to think ahead I am much more creative, know my Bible texts better and have way better stories to tell! I have time to try the illustrations that I want to use and make sure the movie clips fit the direction of the message.
Map it out
Know where you are heading and make sure its clear! I like to get a big piece of paper and write out the title of the night, the main text and my big idea down (see picture below). Once those are clear I can start outlining my messages for the week. When I am able to see it clearly written down I can process it better and make sure I am being crystal clear in what I'm teaching and how I'm asking students to respond to God.
If you don't own it how will anyone else? If you don't know your material, by heart, how can you expect a student to remember it. Now you may think this is harsh or too much to ask but I don't. I always do my best to memorize my material, the flow of the night and how I will close the message. My hope is that if a student ran into my 6 months later that I would know enough of the messages that I preach to still have a lasting conversation about them.
So this is what I'm doing right now; what are you doing so I can learn from you?