Listen thoughtfully to what your classmates have to say. A good discussion is an exchange of ideas, and that starts by hearing what you may not have considered before. Also, repeating a point that was just made shows the class and the professor that you weren't paying attention.
Be respectful in your response. Nobody says you have to agree with your classmates. In fact, you might hear things that make you wonder what planet they are from. But if you really have an intelligent point to make, you should be able to present it without resorting to obvious disgust or name-calling.
Limit the length of your comment so others will have a chance to talk. Though it is easy to get carried away as you passionately defend your view, keeping your remark to the point will be appreciated by all.
Stay on topic. Sometimes, college class discussions have a way of veering away from the subject at hand to off-topic political issues. Though it can be tempting to jump in, limiting your remarks to class material shows the professor that you have done the reading and know what you are talking about.
Respect the professor's procedure for running class discussions. Often, especially in small classes, discussion is open. If your professor likes to call on students, allow yourself to be called on before speaking.
Apologize if you have made a remark that somebody finds offensive. Though it may seem harmless to you, people bring different cultural frames of reference to a class discussion. These differences can become a big opportunity to learn if approached with an open mind.