i've spent eight years in enterprise sales watching people handle disagreement badly in both directions: the yes-men who never push back and get steamrolled or ignored, and the people who make every disagreement a referendum on their manager's competence. neither works.
how i actually do it now, for whatever it's worth:
pick the moment. don't disagree in a group setting unless you have to. you're making your manager defend a position in front of an audience, which triggers defensiveness before the content even lands. 1:1 or a short slack message first.
lead with the goal you share. "we both want Q3 to close strong" before "i think the territory split is going to hurt the number." not because it's a magic softener but because it signals you're aligned on the outcome and disagreeing about the path. different energy.
bring data when you have it. opinions are cheap, numbers aren't. "the last two cycles where we split this way, close rate dropped 8 points in the Southeast" is not an argument they can dismiss by pulling rank.
say what you need from them. "i want you to reconsider" is harder to act on than "i'd like 20 minutes to walk through my concern before the decision is final." specific ask, low stakes.
disagree and commit when you lose. if you push back, they hear you, and they still decide differently, execute their decision well. not passive-aggressively, actually well. this is how you get invited into the next decision.
there's a version of this that's completely different in toxic environments where disagreement isn't tolerated at all. that's a different post. in normal-to-good environments, most managers actually like people who can push back cleanly.