http://cg.tutsplus.com/tutorials/autodesk-maya/3d_cg_vfx_autodesk_maya_mentalray_proxies_proxy_rendering/

In today’s quick tip, Dimitar Katerov will explain what proxy objects are, and how they can be used to render an insane amount of polygons in a single scene. If you’ve ever gone through the pain inducing task of trying to render a scene with a few hundred high-res tree models, then this quick tip may be the answer to your prayers! So fire up Maya and let’s get started!