Don't like your model floating in space? Do you find your avatar falling off the side of the model when you have gravity on? You probably need a ground plane. You may want to bring in an accurate terrain map from Civil 3D but for many of you, you're only dealing with the building itself, in which case a simple flat ground plane will do the job.
The link below will let you download a zip file with 9 ground planes, there's 3 sizes, 10m x 10m, 100m x 100m and 1000m x 1000m, and 3 materials, Grass, Concrete and Asphalt.
Simply Append one to your model and move it underneath the building model, and like magic you'll have a surrounding area to walk around, a better area for visualisations and you'll be able to get a better idea of scale.
For the more advanced users, you can modify the material in Presenter and replace it with a aerial image (e.g. from Google Maps) or a more suitable texture.