Bilateral teleoperation systems are used in different applications to manipulate the environment remotely. The control design for such systems represents a challenge in finding the proper balance in the inherent trade-off between transparency and stability. In order to address that problem, we present a new representation of a teleoperation system that allows to treat separately the mass, damping and stiffness of both environment and operator, allowing to put independent bounds on these parameters without introducing conservatism in the model. Moreover, we use robust control techniques based on Linear Matrix Inequalities theory to design controllers that guarantee performance and stability for a specific range of time varying environment and operator dynamics. Simulations demonstrate the effectiveness of the method to trade-off transparency and stability in a suitable way.