Importance of practice:
To avoid nervousness and anxiety, practice a lot. Candidates usually panic because they feel the panelists or the moderators are out to get them. That is far from the truth. Feel positive for them and you will automatically begin to relax.

How to practice:
Start by initiating discussions amongst friends, become a little aggressive with them and move on to discussions with strangers. Reading from books loudly will help you with your tone. You do not have to shout but a slightly higher tone has a better chance of being heard.

How to get a chance to speak:
The noise level in GDs has a pattern -- there are peaks and troughs. If a GD gets too noisy, try and enter during the troughs. It takes some practice but learn to enter at the troughs.

If you do not understand a GD topic, wait for someone in your group to start. Pick up from what the group is discussing.

Do not read simply for the sake of learning facts. Form opinions with justifications and apply them in practice GDs.