Negative Electron Affinity (NEA) GaAs photocathodes are widely employed for generating spin-polarized electron beams, but their extreme sensitivity to chemical poisoning significantly limits their operational lifetime. This thesis investigates strategies to enhance the longevity of spin-polarized electron sources. First, the evolution of beam polarization during photocathode degradation is characterized both experimentally and theoretically. A novel cesium iodide (CsI) surface treatment is then shown to improve lifetime performance. Additionally, GaAs photocathodes incorporating a robust antimony (Sb) layer into the NEA activation are tested under high-current conditions, where ion back-bombardment dominates degradation. Finally, spin-polarized electron emission from GaN photocathodes is measured and characterized for the first time in both hexagonal and cubic crystal orientations.