You might need more RAM. I recently upgraded from 512 to 2 gigs and it's a huge difference in both posing highly populated scenes and in being able to do multiple renders without having to close Poser.

 I also use Memturbo http://www.memturbo.com/ which allows you to defrag RAM lost from multiple renders and other tasks. This helps in not having to close and reopen Poser all the time.