Never! Interrupt! Always wait for the person speaking to finish. AND, try not to appear like you can't wait for them to finish so you can talk.
Always! Ask questions. Nobody likes a one sided conversation. You will actually find that the other person could very well be interesting. It also helps in keeping the conversation moving.
Avoid! Drifting. Keep eye contact when the other person is speaking. I can totally tell if someone is listening or not by eye contact and their "uh huhs"!
Keep it Clean! Bad language is always tacky. Unless you're a sailor straight off the boat, you might want to think twice about your choice of words. You can make a poor impression really fast with the shortest of words!
It Takes Two! Try not to talk about yourself. It's good to answer questions about yourself or give tidbits but you don't want to come off as arrogant or self absorbed when you it's all about you.
Repeat! Try to repeatthe person's name once in awhile. If they are a new acquaintance it will help you remember their name and it will also help them feel like they are important to you.