The kanji's ON-yomi is the same ON-yomi as one of the radicals in it - usually a STRONG RADICAL. For example, 衣 (cloth) is pronounced "I". 依 (rely on) has the cloth radical and is also pronounced "I"
So if you forgot how to pronounce 依, but you remember how to pronounce 衣, you’re in luck.
Also, I don’t usually bother with an ON-yomi mnemonic for SAME-ON kanji, since you already learned the other kanji and its ON-yomi!

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