Unless your scene is a closeup or you intend to render at very high resolution,  you can reduce your textures.  First, find out where in your Runtime texture folder the given files are located.  My method is to create a folder called Hi Res within that folder, and I copy the textures into it, so as not to lose them.  Then I open up the original textures in Photoshop and reduce their dimensions.  If you take a skin texture that's 4000 x 3000 pixels and reduce it by two thirds and save it as a jpeg, it's going to take up a lot less memory.  Leaving the textures in the original folder will allow Poser to scoop them up and apply them when you choose the mat file.  When your render is done, you can replace the Hi Res textures to their original place.

With figures, before exporting, go into the Hierarchy editor and shut off every body part fully covered by clothing.  Fewer polygons also equates to less RAM.

For importing, I have less problems doing it one item at a time into separate Carrara documents, where I can tweak their shader settings, especially in terms of specularity and bump.  Then I merge them.  Every time you merge something in, you should immediately save, close the file, exit Carrara, then open again.  I've always found that it uses less memory this way than using the file directly after import.

If it's still too much, try rendering in stages.  Plan out where items, figures, and shadows overlap, then add as many items as you can just short of freeze-up, render and save your image, then delete unnecessary items.  Make sure you go into the edit file and choose Delete Unused Objects and Delete Unused Shaders.  Save as a sequential file, and start adding objects again.  The important thing is keeping your lighting continuous.  Then you can take your renders into photoshop, combine them into layers, and simply erase a hole in the layer above to reveal characters and objects.  You can go even further by choosing to render with an alpha layer in .tif or .psd format.  This function is enabled in the render room.

It's rather mind-numbing ardor, but such tricks will allow you to achieve some deft acrobatics without a computer farm.  Hope this info helps.