What phrase suggests that one should not invest efforts into a futile endeavor?

The phrase "Don't saddle a dead horse" implies that one should not invest time, effort, or resources into something that is already ineffective or has no hope of success. It metaphorically suggests that trying to ride a horse that is no longer alive (and thus cannot be ridden) is a pointless endeavor. The imagery conjured by the phrase emphasizes the futility of attempting to engage in an activity that is clearly unproductive or has already failed. This concept aligns well with the notion of recognizing when to abandon an unfruitful effort rather than continuing to exhaust resources on it.

While other phrases highlight similar themes of futility, "Don't saddle a dead horse" specifically captures the essence of engaging with something that cannot yield results, making it particularly relevant in discussions around making wise decisions about resource allocation and efforts in both workplace and personal contexts.