The existing works on histogram-based watermark embedding share the common notions of non-informed random host sample selection and uniform embedding, resulting in limited and yet similar performances. This letter develops two methods to improve imperceptibility and robustness of histogram-based embedding. We first propose a content-aware method which performs uniform embedding in ranked energy-significant regions of the host signal. We show that the embedded ternary watermark signal is more likely to be masked by the strong host signal component, thus improving imperceptibility without compromising robustness. Further, a non-uniform embedding method is proposed, which modifies host samples according to their relationships with the center of the target histogram bin. It ensures minimum host sample modifications without reducing the payload size. Meanwhile, watermark robustness could also be improved since it becomes the most difficult to move samples at bin centers into other bins via attacks. The proposed methods are supported by extensive experimental results.