I know the problem very well - ran into OOMs more times than I care to remember.
Some tricks:

• Avoid dynamic hair. A real resource hog.
• Downsample textures. A 4000x4000 body texture on a background figure is a total waste of memory, 1000x1000 is more than enough. I've batch downsampled all my hires textures to med res and low res (somentimes even as low as 500x500), and made MAT poses for them. 
• Consider using lower poly figures for the background. Posette and Dork look as good as V3 and M3 at mid to long range, at less than a quarter of the polygon count.
• Consider making body parts invisible that won't be rendered anyway.
• Turn off texture filtering
• Lower the bucket size to 16
• Lower the maximum texture size to 1024

And sometimes even all these things won't do the job. Then I'd advise turning to another render engine. Although I've never tested them for myself, PoseRay (free) can convert your scene to a POVRay ready file, and POVRay (also free) is a very capable render engine. I've seen some stunning POVRay renders.

I usually use Vue 6 Infinite for complex scenes such as you describe. My latest render contains 30 human figures, quite a few props, and a fairly complex cave setting. Literally dozens of lights, with volumetrics. Completely impossible to render in Poser, but Vue 6 Infinite 64 bit didn't have a problem.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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