Here are my tips, some of them have already been mentioned.  (I'm not sure if they're all useful, but my intuition said they were!)  Here they are:

Use simple figures when possible.  Poser4 has a few preclothed figures, and Poser3 figures are also simple. 

If creating characters, create a morph and delete the morphs you used to make the morph (not expressions, just the ones you won't be using again).  Poser 6 required for this(?)

Use jpg textures whenever possible.

If you absolutely need shadows, try using multiple shadow lights with very low (64-128) resolutons.  This help will keep your characters to appear on the ground, as well as provide the subtle tones that you miss when turning off shadow maps.

When using shadow maps of any resoution (especially small), remember that Poser calculates the map based on the total area of shadow casting objects.  So if you have two figures standing on the "ground", your render will look better if you turn shadows off for the "ground".

Get a copy of virtualdub and grab as many filters as you can find.  Try experimenting with your preview output.  Preview doesn't support shadows, smoothing (subdiv), or bumpmaps (or most anything else you need for realism) but the images can look great with a little post processing.  I've created a nice bloom filter that, when used sparingly, gave a real sense of atmosphere to the pics I tried it with. 

Avoid things alledged to be "dynamic"

Get a copy of Avid or Vegas and use still renders in your animation.  You can keyframe and output a simple pan and zoom (Ken Burns' effect) animation alot quicker in a video program if you don't need real motion.

Those are my suggestions.  Now I find I spend way more time setting things up than actually rendering them!