Support your concrete model with a firm mold. Your cement mixture must conform to the shape of the mold, not the other way around. If any area of your mold gives way to your cement mixture, it could lead to a warped model. Fiberglass and Styrofoam make sound molds for concrete models, while rubber is better suited for plaster and other lightweight modeling materials.
Eliminate all bubbles in your cement mixture. These tiny pockets of air don't disappear while your concrete model is drying, and they can make your finished product porous and increase its susceptibility for cracking. After you've poured your cement into its mold, vibrate the mold to remove any bubble from the cement mixture. The traditional approach for vibrating mixture mold is to put the mold on a vibrating table. As a cheap, and less effective, alternative, you can place your mold on top of a cloth-covered washer or dryer to vibrate small models.
Add fibers and chipped gravel to your cement mixture to provide your model with reinforcements. Wood or steel fibers add a small amount of elasticity to the rigidity of your model, allowing it to take more of a beating without cracking. Wood and steel fibers won't afford you the strength and stability of rebars, but they'll strengthen your model and they can be much easier to work with in modeling.
While you can use a vibrating table to remove bubble from your cement mixture before it hardens, modeling cement is inherently porous. Paint your model with a cement densifier to patch up its microscopic pores. Not only will the densifier reinforce the model's mass, it will also seal it from moisture and protect it from erosion.