My mom said...
Not getting emotional is the key to disagreeing. If you get emotional then you send a negative message (or a judgmental message). Also not getting stubborn about your point of view. If you get stubborn you come across as arrogant--not good. If you are gentle about your disagreement then the other person can absorb the message and not the arrogance.
My mom says that I am good at disagreeing with other people b/c I follow those rules.
I think my mom is good at being a disagreed with, she listens and followes the same rules.
we make a good pair :)