All the above suggestions are good. But what I find is most frequently the limiting factor is the amount of RAM used by the texture maps. If there are several figures in your scene with clothing and each figure uses even medium resolution textures, then your RAM will disappear very fast. If the figures are small in the scene it is a waste of RAM to load a 3000x3000 head texture for each. You can limit maximum texture size in the Render Options panel to anything you want. Poser will rescale the textures and just use the RAM required for the smaller size - this can often make the difference between whether a complex scene will render or not. I once had a scene with one figure and about a hundred flowers that would not render. The figure used moderate res textures and I did not want to reduce them. I examined the flowers materials and discovered that each Petal of each flower used a 512x512 map. That may seem small, but multiply it by a few hundred and it is a lot. I used a batch processor (Irfan View) to resize all the flower maps to 256x256 (which reduced the RAM requirement by 75%) and the scene rendered fine.