How Do Consulting Firms Handle Performance Appraisals with Multiple Managers?

Hi All, I need some information on the best practices in performance management systems in consulting firms. Basically, I'm part of the HR team of a software development firm, and we have an annual appraisal cycle. Our employees work on projects for different durations, ranging from 3-4 months to over a year. Consequently, they report to different people over the course of a year. I would like to understand how this process is managed in other consulting firms such as PriceWaterhouseCoopers and other IT firms that follow a similar approach. How do we gather feedback from various reporting managers and consolidate it?

Any information or suggestions would be greatly appreciated.

Thanks! Deepika.