If you are appearing for an interview then following tips will help you to handle GDs effectively.
You should be audible and clear first. Remember you are participating in a discussion which is different from a stage speech. Even if you disagree with others' point of view put your argument politely. Don't argue with loud voice.
Be a Communicator
GD means all participants are free to express their views. Depending on the topic, the points should be short and precise. You should felicitate the conversation of others also. Respect others' opinions also. Try to moderate the discussion if any argument arises.
Establish eye contact with other members of the group. Don't let your voice tremble. When sharing your points you should look at everyone. And if someone else is sharing his views and you agree with it. You should nod. Because it shows that you are receptive.
When you listen actively. You will be able to find two or three points familiar to you and could build on that.
Use Quotes/ Statements
Try to use facts, quotes or statements. When you use such things in a group discussion it depicts your thought process. It shows you are well read. You are globally aware. And it also shows a clear chain of thought.