Negative space scares the crud out of many scrappers. I used to fear negative space as well. It can be so hard NOT to fill all the area on that large 12x12 space. The goal is to make something that is pleasing to the eye with out it looking crowded or not finished.
I think the first step in creating a negative space layout is to focus on one subject. You need to "ground" that subject/photo so it does not appear to be floating, but it is an intended design tool. I think to do this you need to "anchor" your design to a side or the top/bottom of your page. Some people are very good at placing the subject in the middle of the layout. I have yet to achieve this. So I stay near the sides or the top/bottom of my layout. Of course this all just preference.
In the sketch and layout below, I have achieved a pleasing negative space layout by anchoring my subject to the bottom of the page, and allowing the top to remain free. This brings the focus directly to my subject and the feeling I am trying to express.
Please feel free to try this sketch or make your own layout using negative space. I think you will like the results.