I really don't think there's any major difference in preparation required for ZS vs. any other boutique firm (and before you ask - yes I have helped clients get into ZS).
1. Practice standard cases, like you will find everyone here doing
2. Make sure you are also solid on the mental math; great resource is caseinterviewmath.com; another solid resource is found here on PrepLounge
3. Make sure also you are up to speed on how to read an exhibit or chart. There is a strict 3-part methodology I always recommend and which helped me get through my 1st round at BCG: (1) what am I looking at, what's the exhibit talking about; (2) read the data, what's the story; (3) what's the insight, what am I going to do whith this, what is my new / refined hypothesis.