It depends on if you're arguing to prove a point or if you're arguing to be a smart-ass. Either way, here's what I'd recommend:
1. Don't dismiss the validity of your opponent's claim(s).
2. Warrant every argument you make. Specific examples (historical, current) increases the credibility of your arguments. As do smart analogies!
3. Point out the inconsistencies in your opponent's logic.
4. Make sure to answer each point that your opponent makes. Good debates/arguments have clash.
5. Depending on what you are arguing about, you'll want high quality evidence to back up your points. What is high quality evidence? What makes an expert? It is your job to prove that your evidence is relevant.
6. Don't ramble and rant if you can avoid it.