Ice needs a couple of things to look good. It is strongly reflective and strongly refractive. Accordingly, both of these elements should be light gray.
The reflections and refractions can not be too perfect, except for ice that has been dipped in hot water. This is where some Glossiness maps come in: they create the much-needed imperfections in the shader.
A nice, noise-type Bump map is also needed to make the surface less smooth. A couple of differently-sized noise maps, composited together, will work well.
Finally, make sure that the Reflect on Back Side is turned on.
This gives us a clear ice shader. To add some frost to it, we need to use Translucency. Set it to Hybrid mode and use some sort of Fractal noise in the fog slot. Control the frost amount with the Fog multiplier. You may need to bump up the Light multiplier as well, to get a realistic result.
