(Trying to get WordPress to put images where I want them is always a struggle when I have more images than text, so I apologize for the presentation.)
In one respect, building a gingerbread house is like any other construction project: you need to start with a plan.
(You can click on the images to enlarge them.)
Many gingerbread books include plans for the structures they feature (some do not). There are also some available online. You can also create your own by constructing your buildings out of cardstock or cardboard, then using the finished pieces as templates for your baking.
Part of our prep work also includes making modeling chocolate. Domed and round structures are really hard to make out of a cookie, so using a flexible medium eliminates a LOT of stress.