Step 1: Test the alternatives: "by heart" belongs with "learn" or "know," not with "take." "In heart" is not an English idiom. "At heart" means fundamentally or deep down, which changes the meaning entirely. Step 2: None of these substitutes fit the intended sense of being emotionally affected by words. Step 3: The original phrase "take my words to heart" is idiomatically correct as it stands. \[\boxed{\text{No improvement needed}}\]