We report on the preparation and operating conditions of negative electron affinity GaAs, cesium-oxygen activated photocathodes, cleaned using atomic hydrogen generated by a thermal cracking source. Atomic hydrogen is effective in removing surface contaminants and in producing a smooth reconstructed surface free of oxides and carbon compounds at a relatively low GaAs temperature. A clean (2x4) reconstructed GaAs(100) reflection high-energy electron diffraction (RHEED) pattern is obtained after atomic hydrogen cleaning. From the RHEED patterns, the thickness of the cesium layer that causes maximum photoemission is estimated to be similar to 0.5 monolayer. High quantum efficiency photocathodes (similar to 12%) are produced after activation to negative electron affinity. Atomic hydrogen cleaning is effective not only in the initial cleaning but also in reviving the photocathode after its performance is degraded by operation. The GaAs is kept at 500 degrees C during the cleaning process, a temperature that preserves the arsenic stabilized surface necessary for negative electron affinity activation. By reducing the accelerating voltage of the photoemitted electrons and the photocurrent intensity, and by keeping a low level-of continuous cesiation, the photocathode lifetime can be significantly enhanced.