Sketches and Photoshop mock ups help a great deal in planning the Halloween House. Sometimes, though, I find it hard to envision how the scene will work as those designs translate into 3D structures and how the overall environment will feel in relation to the size of the visitors. So prior to building structures or finalizing the dimensions of pieces, I like to create a model of the whole scene (with 1 inch on the model representing 1 foot in real life).
I have a model of my house (with the support structure overlaid) which I made out of paper and foam core. This acts as my stage for experimenting with pieces of the front yard experience.