1. A good conversation is never about one person.
If I'm talking too much about me, just interject and draw the conversation back to you. And expect me to do likewise. If I notice myself (or someone else) dominating the conversation, I'll pause and ask someone else a question.
2. Never interrupt someone else's story.
(Unless they violate rule 1.) If you only have a few minutes and there's something you're dying to tell, go ahead--but that isn't really a conversation. It's more of a confession. In a conversation, you're probably spending several minutes, maybe an hour, with the same set of people. Expect give and take. There will be time to bring the conversation back around to your story.
3. Avoid starting or ending with something serious.
Conversations have a natural rhythm and flow. The first bit gets the conversants comfortable before the real work can begin in the middle. But don't wait too long for that moment! There's a point when people want to withdraw, and then it's too late. If people are yawning or talking about plans for the morning, those are signals that they want to withdraw. Also check to see if they are looking around for other people to talk to. No matter how much people like you, there's a limit to how long they want to converse.
4. Bring something to the conversation.
Sometimes it's difficult to know what to say, especially to someone you've just met or someone you see all the time. That's when you draw on your prepared topics. What have you done recently? Have you eaten at a good restaurant? Heard an interesting new song on the radio? Started a new gym class? Saw something funny on the internet?
4a. Elaborate and help out.
I offer this classic interchange from L.A. Story:
Trudi: Sheila has been studying the art of conversation.
Harris: Oh, you're taking a course in conversation?
What other advice would you offer?