A useful tool I (try to) remember is the following:
I believe / think / understand: For expressing an opinion based largely in personal experience and lacking "credible" evidence <insert Cancer Protocol argument here - I'll detail at the end>. Largely has no scientific references or reputable sources that can be quoted, e.g. while Sam Ramsden up the road might write a blog that agrees with your anti-toenail polish stance, the chances are you won't find agreement in New Scientist. These statements are written from the poster's perspective.
It is known / proven / has been shown: For expressing a demonstrable "fact" (given that science is a constantly changing and evolving field and accepting that what is known to be "true" now may not, actually be correct: Flat World Syndrome). To be polite and to demonstrate credibility, it's good to find a reputable source for your information.
Accept someone else's reality: Basic Theory of Mind stuff, but still easy to forget. Just because the sun sets at 4pm for you, doesn't mean it's dark everywhere else in the world. People construct their own realities from their own experiences; no two are the same, and no-one has the right to disclaim or deny someone else's "reality", no matter how hockey and bonkers it might seem (see: anything about religion, politics or alternative medicine).
I am (you may have noticed!) highly opinionated and very vocal about my thoughts (though I try not to be aggressive!) and I learned these three forum "rules" the hard way. But in my years of opining, they're the three rules that best help a community rub along nicely.
*The Cancer Protocol Argument:
This is an extreme version of the "It worked for me!" argument. Essentially, new cancer protocols are measured in "success rates" which (brutally) translates into "How many people died before this one patient survived?". That's the objective view. If you are that one, single patient for whom the new protocol was successful, you'll swear blind by it, because you can only see things subjectively: you won't know that you're Patient 15, and you won't really care. However, the impact the treatment has had upon you - i.e. miracle cure, saved my life etc - will absolutely convince you that this new, amazing treatment worked for you, therefore it will work for everyone. (You don't have the data, but you do have the evidence)
Both subjective and objective arguments are important, but it's also important to be clear about the differences between them. The Cancer Protocol Argument is just an extreme example of why.
