You will have to pursue the following points if you want to be successful in a group discussion. The points are as follows:
Make sure that you read as much news as possible. Generally the topic is chosen from some of the current happenings. Hence you should make sure that you have a look at all of them.
Make sure that you speak loudly and clearly. This is one of the most important things and you could find yourself in difficulty if you do not speak loudly and clearly in a GD.
You should know that you are not really bounded to think in one way. You are free to think on your own and put forward some new ideas. It is good for your chances and also for the GD since new ideas will definitely make it interesting.
You should know what the topic is really about. If you have some confusion then you should wait for some time and let the other speak at first. When the matter is quiet clear and the GD is in midst then you should start speaking with new ideas in mind.