You really need two surfaces for glass and air surface and an glass surface. Take your glass surface copy it and paste it in another layer. Flip this surface and name it air. Paste it back in the glass layer and merge points. Then try these settings for starters.
http://www.geocities.com/CollegePark/9315/lw/tip4.htm
Philip also sets up a glass surface in this free tutorial. You can check out his settings he used here.
http://liquid-arts.de/techniques.htm
http://liquid-arts.de/tech/JD/JD-05.htm