Reflection + Refraction + Diffuse must equal 1 (or in DS terms, 100%)...the exception to that rule is Diffuse set to Black (0,0,0)...100% Black is still 0. Otherwise, you will get more ‘energy’ out of the reflection than goes into it, which is the most common cause of washed out reflections.
I got pretty poor results with anything other than diffuse at 100 percent doing testing the other day. That is for any situation where a color was wanted other than traditional silver mirror. I also suggest setting specularity at 0 for any flat surfaces like mirrors, but only flat surfaces.