Know what you are talking about. It's hard to sound convincing if you don't understand the subject matter.
Keep your voice even. Shouting does not prove a point.
Be open to another perception. It is okay to concede a fact or point or even see the other person's point of view.
Be willing to stand behind your conviction. Do not sway your opinion if you don't believe in it. Be willing to present creditable reasons to support the way you think about the subject.
Do not be afraid to admit it if you find your judgment in error. It's okay to be wrong. all opinions and beliefs are based on the information an individual possess. Sometimes the information we posses is outdated, misconstrued, or misrepresented. Standing corrected enlightens and does not diminish.