I loved my day at Chester Zoo recently:
When shooting against glass if you have a lens hood try and get the hood flat against the glass to reduce reflections.
Try and find out feeding times, some of my favorite shots were at the end of the day when I happened to be near the meerkat's and they got fed mice, totally different shots to the usual 'sentry' shots.
Look for the good light and try waiting for an animal to go there instead of just shooting it where it happens to be at the time.
Two that have stuck with me recently are: look both at the forground and background when taking a shot, and 'whens the best time to shoot portait? straight after horizontal'
See through your own eyes, not just the viewfinder, I work at Knowsley and sometimes take the animals for granted but just spend time watching them instead of taking pictures all day too