Don't let somebody else determine your behavior. What does it mean?
In the following text, What is the meaning of the last sentence(Visible~behavior.)?

4. Trust is the most important factor
Most negotiations take place in a climate of shared expectation. We expect that when we give in a little, the other side will give in a little, too. We expect that if we bargain in good faith, so will the other side.

"Visible good will is the strongest negotiation stratege. Don't let somebody else determine your behavior."

Question

Don't let somebody else determine your behavior. What does it mean?
In the following text, What is the meaning of the last sentence(Visible~behavior.)?

4. Trust is the most important factor
 Most negotiations take place in a climate of shared expectation. We expect that when we give in a little, the other side will give in a little, too. We expect that if we bargain in good faith, so will the other side.

"Visible good will is the strongest negotiation stratege. Don't let somebody else determine your behavior."

Answer

"visible good will" refers to acts of kindness that are public. Holding doors, being polite, helping people with various things, etc.

From the second sentence, I understand this:

Even if someone else is unkind to you, don't be unkind in return. In other words, don't let their behavior (being unkind) determine your behavior.