in general, I mean it's hard to state your opinion in a diplomatic way without sounding opinionated or defensive.
Especially when you are challenged haha
Most Helpful Guy
Hmmm... by making pauses, and sounding as if you're not totally certain of what you're saying, always leaving room for improvement. By using non-committing words such as "sometimes" or "maybe", or something like "I'm not sure, but..." leaving an open end for discussion and doubts. When they get back on you, you don't need to get defensive and say "I said MaYbE!!!". But rather repeat the same ideas, with the same non-committing tone.
This way, your point will still get across, and you don't need to state that you're a firm believer of something (even if you are), and by using these words, the other person won't attack you (hopefully), and may give you criticism that can help you to get some reasoning behind your ideas/decisions.1
Most Helpful Girl
It's easier when you avoid using extreme or overly descriptive words. And also when giving an opinion about a person, describe their behavior, not them as a person. For instance, instead of calling someone a harsh name like jerk, asshat, etc, say they're were unpleasant, disrespectful, uninformed, etc.
Also avoid using all or nothing verbiage. So often I see people refer to "women" in general or "all women". The truth is the world is filled with so many countless different people that never could every single person of a particular category (gender, race, religion, sexual orientation, political preference) have the exact same opinion, behavior, whatever. Just yesterday I saw someone write about how all whites are racists. That statement couldn't be farther from the truth. And I've read repeatedly that using that kind of language is detrimental to a relationship... that includes relationships of any kind, romantic or otherwise.1