You can minimize the dark areas to some extent by playing around with global illumination (lots of lights) and the Ambient values of the materials.

A different approach (which in my opinion looks better) would be compositing in post. Render the figure normally (without making it transparent) against a blank background, then load the image into a paint program like Paint Shop Pro or Photoshop and use its alpha channel to composite it against a new background. By playing with the transparency values of each layer, you can make the figure as ghostly as you like. It gives you better control over the results.

Read this thread for details and illustrations.