Do

Stay calm. If you lose your temper – you lose.
Use facts as evidence for your position.
Ask questions.
Use logic to undermine your opponent.
Appeal toworthy motives that are hard to disagree with.
Listen carefully.
Be prepared to concede a good point.
Study your opponent. You can appeal to their higher values or exploit their weaknesses.
Look for a win-win. Be open-minded to a compromise position.