Just a couple of comments:

1) Poser doesn't need normals.  In fact, don't include them. Scrub them from your object.  It's a waste of space and can cause problems.

2) Booleans are evil.  Poser usually craps up when encountering meshes that have had certain boolean operations performed on them.  The resulting mesh, frequently lots of thin triangles and bajillions of edges, is just "Bad" for Poser.  If you can avoid using Booleans when creating objects for Poser then do so.  If you can't, then prepared to go in there by hand in order to fix them before you polish off the model.

3. Single edged edges are something Poser has trouble with and frequently craps up on when trying to render.  Bevel your edges, even just a hair, and Poser is much happier with them.

4) IMO, big unbroken flat planes are something Poser doesn't like either.  That seems counterintuitive and may be completely wrong, I dunno.  But, on something like a long wall, I just break it up into a few polys anyway and seem to usually avoid weird shading problems.