You are trying to mimic the effects of soft, diffused natural light. Professionals use soft boxes to get rid of shadows and harsh reflections. These are little tents to soften the light without diminishing the intensity. You can easily build your own to scatter the light and soften it up.
Diffusion materials are translucent papers, films or fabrics that scatter light without diminishing the light's intensity.
- Fabric: You can use translucent blends or a translucent shower curtain with a matte finish. Be careful when using a the shower curtain that the heat of the lamps doesn't melt it - it is plastic. Photo supply shops have special diffusion fabrics that are inexpensive, called "silks." These diffusions photo silks have the advantage of being flame retardant and won't melt on hot lights.
- Gels: Diffusion gels are available cheaply from photo stores (Rosco offers a broad range of light diffusing materials on Amazon.com). Called "light diffusing material," they run about $6.00 for a 20" x 24" sheet that you can cut to size. They are translucent and flame retardant.
- Vellum and Tracing Paper: In a jam you can always use tracing paper or vellum, but be careful! Tracing paper and vellum can melt or catch on fire.