Effective communication is not just about speaking. It's about how to deliver your point in so many ways. If you aim to be positive in your communication in dealing with people, you will need to deliver effective communication. Sometimes we don't realize how we communicate and the signals we are sending people.
Regarding communication skills, eye contact is basic. If you are not looking at the person when you speak, you are sending a message. Whether you are distracted or disinterested, is not the issue. It is downright rude. If you've ever taken any kind of public speaking course or remember it from high school, eye contact is an important part of the course. This goes for one on one communication as well. Communication on the personal level is utmost important. Some skills are learned ones and don't come naturally. We also pick up bad habits and get lazy in our skills. This can really harm relationships. If you are a good communicator you will not send undesirable signals to the other person.
In all relationships, good eye contact while in conversation is crucial. It will make or break your relationships. It sends a definite signal to the other person, negative or positive.
Sarcasm is negative communication, not effective communication. It will kill your relationship. It means there is something going on with you, an emotion, a vendetta, or complex and that you are taking it out on the other person. I am not talking about humorous sarcasm but blatant, rude, careless sarcasm. This should not be used with people you like. You will surely lose them.
Don't build a wall between you and the other person. Open communication has no barriers. This doesn't mean you will always agree at all. This means that it is open for discussion. It doesn't need to get ugly and it's ok to be firm when you don't agree.
Don't get angry and say negative things. If you can't refrain from this behavior, leave the room and resume the conversation at another time. Learning to control negative feedback towards others is a skill we all must learn in life.
Body language can send signals to the other person or persons as well. Waving your arms around or becoming violent shows you have anger issues. If this is the case, you need to seek counseling to resolve the issues that are not yet resolved. Rolling your eyes can also have a negative effect on relationships and will not promote effective communication. Paying attention to the ways in which you communicate can help you change the negative one that you are using.