The phrase "losing luster" indicates a loss of brightness or shine, or a reduction in quality. Antonyms for this phrase would include terms that convey brightness, excellence, and attractiveness. Synonyms of "losing luster" such as dullness could be contrasted with opposites like brilliance, radiance, or resplendence. Words such as decline, degradation, or deterioration could receive antonyms such as enhancement, amelioration, or rejuvenation. The opposite of loss could be gain, so phrases like "gaining shine" or "boosting quality" could also function as antonyms for the phrase "losing luster." Ultimately, the antonyms for "losing luster" would imply an improvement or recovery, such as a return to a state of excellence or a newfound vitality.