I’ve seen some debates, and they come from one topic, to a whole different topic, what you need to do is STAY ON THE TOPIC GIVEN. When changing the subject it leads to spamming, look at the Darwin Evolution thread.
I’ve developed a little system that will show you to stay on topic, and have a good time while you’re at it, without taking the risk of getting the topic locked.
Define - What is the topic actually about? The topic always has a meaning, unless the topic is spam. Defining the topic will give you the response you need that most people cannot pass by, or prove false. When defined, making a response is easy.
Choices - Which side are you taking? For example, say a thread about President Bush’s decisions is made, are you with Bush or against him, or are you neutral? If you make your response seem way off topic like for example, “Bush is from Texas, and so was one of his top workers that has recently resigned.” It’s very neatly evaluated, but on the wrong topic.
Opinion - Make sure you don’t use opinion too much, that’s what gets topics trailing off-topic. For example, “Bill Clinton gave military secrets to China, I think that he’s stupid. Now I think America is stupid, and is taking control of the world.” If the topic is about Bill Clinton, keep it with Bill Clinton, or it will go to spam.
Evaluate - Do you think you made the correct decision? It’s not too late to change sides, facts can make you change your decision. Just make sure you use facts and don’t go off-topic while changing opinion.
I hope I’ve helped, and hopefully, topics won’t be locked as much from immaturity. 
Don’t get mad at me for making this, it’s just kind of like a “Debating for Dummies” packet.