بِرِعَايَةِ جِهَاتِ حُسْنِ التَّشْبِيهِ
By observing the features that make a simile effective.
The primary criterion for the excellence of realized and representational metaphor is the quality of the underlying simile. Although the comparison is not overtly stated as a simile, its ground of resemblance still links the intended subject with the borrowed image.
A fine metaphor therefore requires more than an unusual word: it combines a fitting relation, fulfillment of the rhetorical purpose, and an elegant mode of presentation. Difficulty or rarity alone is not proof of beauty.