Practice active listening
I think the wisest piece of advice I can give to anyone wanting to improve any type of relationship is to listen better. We're all in such a hurry to get our next sentence out that we don't really listen to what people are saying to us.
I think someone can sense when you're not really listening and it hurts the conversation on some level.
The next time you're having a serious conversation with your friends, your boss, your spouse, try to focus on listening and don't worry about what you're going to say next.