If you're using Poser 8 or Poser Pro 2010 you can up the ambience in your shaders and use IDL to render so the 'ghost' will illuminate close objects or alternately position a point light in your ghost figure and use inverse sqare falloff on that light to do the same thing even without IDL.  

You may also want to try other ghost shaders.  Go grab Ajax's P5 Shader pack, he's got some great 'ghost' and 'spectre' materials you might find useful.

http://www.renderosity.com/mod/freestuff/index.php?user_id=44709