Best Practices In Performance Appraisal Process

Hi All,
Need some info on the best practices in the performance management systems in consulting firms. Basically I'm a 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 of say 3-4 months to more than a year and hence report into different people during a span of a year. Need to know how is it done in other consulting firms like PriceWaterHouseCooper and other IT firms wherein similar process is followed. How do we take feedback from different reporting managers and then collate it.
Any of info/ suggestions would help.
Thanks!
Deepika.

In the PSU where i work as a HR manager , we have a 3 month minimum period for generating an APR. If there are more than one such reports we average it .