There is a popular wisdom that what other people think of me is none of my business.

That's a good recipe for resilience; but in my own behaviour towards others, it is worth living by the maxim "don't say anything about someone that you would not be prepared to say to them".

This applies to both bad and good opinions. Especially good opinions - I always like to copy people in on good feedback I send to their boss, their company, etc.

Negative feedback is arguably more difficult to deliver personally.

Of course there are times when we like to complain about others.

It also takes some sensitivity to deliver a negative opinion to the person.

But maybe if I held myself to a standard where I would have to repeat something I said about someone to that person, I might think twice before moaning about them?

I might consider where they are coming from first?  

Not easy to live by. But worth striving for.