mirror reflections work best if you set Fresnel to 0.0 and the Reflectivity higher then you would in a regular render
for better outer reflections add an image texture containing an environment map containing a simple sky and ground and set its mapping to Reflection, its influence to Color and the Blend to Screen.
render baking is way slower then regular rendering, so try to avoid the expensive mirror reflections with a gloss amount lower then 1.0
render baking does not use anti-aliasing! That's why you need higher sample rates for ambient occlusion, Area-Lights, Ray-traced soft shadows and things like mirror reflections. It's best to post-process the render baked images with a image-based antialising in your Image-Editor to reduce noise in the texture and to be able to reduce sample rates during render baking.