how to argue effectively
Stay calm and keep a steady low tone
The other person will sense you mean no harm if you are calm, therefore they relax. Avoid high-pitched sounds, try to sound as relaxed as possible and don't lash out unexpectedly. Have a good firm posture that makes you feel powerful but not aggressive.
Don't insult the other person
People are much more sensitive when they are angry and will take even the smallest comment as an insult. Try to avoid saying anything that could be taken the wrong way as an insult.
Be reasonable and willing to listen
Respect and listen to the other person. Never have the expectation to get everything you want.