It largely depends on the "glass" that is used in the lenses.

If the glasses allow me to change the material for the lenses, I usually use the glass material from templargfx's car materials freebie.

Make sure you have raytracing turned on. 4 bounces is the usual.

http://www.renderosity.com/mod/freestuff/index.php?username=templargfx

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi