Pay attention to reflections on the glass. Move from side to side, or up and down to make the glare go down. Also, the closer you are to the glass, the less likely you'll see a glare. Also, when in a place like a herpetarium where the enclosures have to be warmer for the snakes, watch out for water on the glass from the humidity.
I love the zoo. I wish I could spend an entire day there. The herpetarium, especially, is a good place to get some great shots. This snake was coiled up in a corner. I got very close to the glass and got a quick shot. I especially love how it focused on his face and blurred his body.