Heres what i was told the first time i was on a buddy box...im not an instructor yet, but my instructor (the initial one at least) set me straight from the beginning.

#1 rule: My word is law. If you argue about me taking control of the plane, I will let you win that argument the next time you get in trouble.
2. I have my planes to fly too. I know your excited about flying, and im excited to help you, but just remember that I would like to fly to.
3. He clarified the right/left situation
4. Communication, its important for us to know whats going on. (he wanted me to acknowledge him, like when he said "I have it" i said "You have it" and vice versa, so he knows that I know whats going on.)
5. He made sure to let me know that the second i felt uncomfortable with what i was doing, to let him know...and we could take a step back and build faith in my skills (never had to do that ... i went fast hahaha)

I think one of the best things about a good instructor is being frank about things. Ive heard other instructors go "well, it was maybe too windy for you" or "maybe you werent ready for that" ... mine was "It WAS too windy, you should have told me you couldnt handle it" ... not so nice, but gets the point across that I am ultimatly the one that decided when i fly.

One of the check pilots where i fly now is like that. An older gentleman that flys at another feild wanted to get checked off at this one. It was windy, 2 planes had already crashed from crosswind landings...but he insisted so the check pilot let him go. He was out of control 3/4 of the time and almost crashed hard twice. After barely getting it down (broke a prop) he went back and started fueling up again. He wanted to go again, he was saying that he was just very nervous about being watched so close...so the guy let him try again. Telling him that it might be better to hold off and let the wind die a bit...but he insisted. So up he went. And down he came...broke the plane in half. Easily repairable...but still...

After the flight our check pilot was explaining that you should know you abilities, and not fly outside of them. Some of the guys thought he was being rough, but i appreciated the fact that he was being brutally honest...because at least then the right message might come accross rather than the padded pity speech that alot of people give, and the the student or whoever doesnt really get the idea behind what they did wrong.