Turn off shadows for the lights. 
I render preview animations using three spotlights , all ligt grey color, spaced at 120 degree pointing at the center where my figure is rotating. Positioned slightly above and pointing down about 30 degrees. Spotlight start angle 70 , end angle 140 .

With this setup you will still get some small specular changes as the model is turned.

With Poser 7 or 8 you could use Image based lights or GI.