The cryogenic subsystem for the Advanced X-Ray Astrophysics Facility (AXAF) X-Ray Spectrometer (XRS) represents a new breed of long-lifetime, hybrid stored cryogen system. The design achieves a predicted lifetime of 5 years with only 480 I of superfluid helium by using mechanical coolers to limit the parasitic heating into the Dewar. The system uses two momentum-compensated pairs of split, Stirling-cycle coolers to cool the Dewar outer vapour-cooled shield. Use of the 80 K coolers can reduce the helium loss rate by about a factor of 3. This paper primarily addressed the means for making efficient use of the coolers. Thermal efficiency is achieved by minimizing the parasitic heating and thermal impedance of the cooler-to-Dewar interface within the mechanical constraints. The important issues related to compatibility with the instrument system are also briefly discussed. Parasitic heating from the non-operating (reserved or failed) cooler pair can be reduced with an active thermal switch that isolates the cooler cold tips from the cold finger. Analysis was performed to determine optimal switch operating characteristics for this application. Requirements associated with use of the coolers, analyses performed and the interface design concept are discussed. Component development is continuing.