We fabricate suspended molybdenum disulfide (MoS2) field effect transistor devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3-4 orders of magnitude. Mobility of the suspended devices ranges from 0.01 to 46 cm(2) V(-1) s(-1) before annealing, and from 0.5 to 105 cm(2) V(-1) s(-1) after annealing. Temperature dependence measurements reveal two transport mechanisms: electron-phonon scattering at high temperatures and thermal activation over a gate-tunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates, but likely by defects, charge impurities and/or Schottky barriers at the metal-MoS2 interfaces. Finally, this suspended MoS2 device structure provides a versatile platform for other research areas, such as thermal, optical and mechanical studies.