Research on wireless sensor networks (WSNs) has recently moved from the design and evaluation of protocol stacks to real-life implementation and test of the proposed solutions. It would be desirable to be able to run the implementation of a given protocol stack (or of multiple stacks) over different real-life test-beds obtaining comparable results. This requires standardized interfaces to remotely access and control a federation of test-beds, tools to gather data on the protocol performance without affecting the network operations, as well as ways to standardize how data needed to compute the metrics of interest are gathered and analyzed. This paper describes the design and use of JAMES, a new Test-bed Management System for WSNs that addresses all these challenges. We also discuss the use of JAMES features to ease the experimental evaluation of protocol stacks we have developed. Our experiments show that only a few tens of lines need to be written to be able to execute tests on a given protocol stack and that JAMES operations are transparent to the stack behavior.