Hello dappledesigns! I love taking pictures at the zoo. For most people that is one of the few places you can take pictures of wild animals. Here are a few tips the I will give you, however, I have recently written a hub on taking better pictures at the zoo. I would be happy for you to read my article and get your opinion on it. It has really done quite well.
The first thing I would recommend is, if you have a camera with zoom capabilities, that would be the best camera to take to the zoo. You want to get some good close up shots of the animals. You have to watch the "junk" in the background also. You want to crop out things like the fences of enclosures when possible. Take some wet wipes, such as Windex wipes to clean the glass on some of the enclosures before taking pictures through them.
There are many more tips I could give you, however, my hub will expound more on my tips and there are some pictures also. You can find my article here on HubPages. It is titled How to Take Better Pictures at the Zoo.