Flutter is an unstable oscillation caused by the interaction of aerodynamics and structural dynamics. It is current practice to operate aircraft well below their open-loop flutter speed in a stable flight regime. For future aircraft, weight reduction and aerodynamically efficient high-aspect-ratio wing design will reduce structural stiffness, and thus reduce flutter speed. Active control of the flutter phenomena can counter adverse aeroservoelastic effects and allow operation of an aircraft beyond its open-loop flutter speed. This paper presents a systematic robust control design method for active flutter suppression. It extends the standard four-block mixed sensitivity formulation by a means that targets specific dynamic modes and adds damping. This enables a control design to augment the damping of critical flutter modes with minimal impact on the rigid-body autopilots. Finally, the design scheme uses a manageably low number of tunable parameters with a clear physical interpretation. Tuning the controller is hence considerably easier than with standard approaches. The method is demonstrated by designing an active flutter suppression controller for a small, flexible unmanned aircraft; and it is verified in simulation.