I frequently photograph sunglasses for various clients. The challenge is once-again in reflections. A small reflection is not really an issue since they are glass and do reflect. Often it is a matter of photographing the glasses in a white studio to minimise reflections and then using photoshop to retouch the lenses.
The background needs to be minimised as well. Often this is achieved with photoshop using a curves layer to blow-out the highlights and then masking the glasses back in.
A small amount of post production goes a long way in product photography.