i don't know if your model is capable of good results with this, but using refraction will add the effect of glasses "bending light". add a bit of reflection to it as well, and possibly specularity depending on previous results. 

basicly look up realistic glass shaders, or read the recent thread about gems. glass and gems effect like in the same way, just need a different refraction index, to get decent results. unless you plan to go for extreme realism, which i won't go into, lots more work :) but this should get you started