I keep my mock-up at my bench at all times during the building process. This helps me to keep my objective always in mind. Photographs can also be used in building mock-ups. When scaled to the proper proportions it is very easy to use a photo as the mock-up. This can be very useful if only one or two sides of a structure will be viewed.
Photos have a lot of limitations as they can only really be used if the photo is taken straight on.
I build my mock-ups from discarded cardboard. I try to collect solid cardboard, not corrugated. I collect a lot of this material from coworkers' note pads when the pad is finished. If you have a plan already copied to scale it can be used to build the mock-up. The plan should be arranged on the cardboard so that the entire structure is connected for easy folding and as few cuts as possible.