Simply make sure your scene doesn't use more RAM than necessary: Keep texture map at reasonable sizes (no need for a 4096x4096 map on a background prop), hide/disable invisible body parts (e.g. hide the hip if your character is wearing pants), turn shadows off for lights that don't need them, keep your shadow map sizes as low as possible (instead, adjust the shadow lite cams) etc. That'll not only reduce the amount of RAM your scene needs but most likely speed up both preview and rendering.