Use your word limit well. A newspaper opinion/comment piece of about 800 words gives you room to make only one point clearly.
Start with your main point or question: the first couple of sentences may be your last chance to hook your busy readers and draw them in. Convince them that this read is going to be worth their while.
Show them why they should care. Every opinion piece should anticipate the reader’s question: so what? What does this have to do with me? Why should I care about it? It may help them understand, if you tell them why it matters to you.
Showing is usually better than telling. In other words, you can often help your readers understand an abstract argument by using an example, an analogy or a quick story.
Think, too, about ways in which you can use images or other media to help make your point.
Write in short sentences and paragraphs. For this exercise you don’t need to follow the conventions of the academic essay. Topic sentences and long paragraphs have their place when you’re writing for other academics, but the aim of this exercise is to get you to communicate what you’ve learnt at university to a wider readership.
Avoid jargon where you can. Every field of knowledge has its technical language. This can speed up communication between experts. But while it’s good to be brief, you also need to think about your non-expert readers. If you’re not sure whether they’ll understand these words, can you replace them with simpler ones, or do you need to explain them?
End well. Readers who don’t know whether they want to read a column often read the first and last couple of sentences to see if it’s worth spending more time on it.

Some examples:

Ali, Zain. “Don’t throw out Baby with the Holy Water.” New Zealand Herald, March 13, 2014. http://www.nzherald.co.nz/religion-and-beliefs/news/article.cfm?c_id=301&objectid=11218524
Brown, Andrew. “There’s No Such Thing as an Atheist Baby.” Guardian, June 12, 2014. http://www.theguardian.com/commentisfree/2014/jun/12/atheist-baby-richard-dawkins-babies-atheism
Malik, Kenan. “Myanmar’s Buddhist Bigots.” The New York Times, May 19, 2014. http://www.nytimes.com/2014/05/20/opinion/malik-myanmars-buddhist-bigots.html
Whitehouse, Andrew. “Religion, Children and Doing Unto Others.” The Conversation, July 9, 2012. http://theconversation.com/religion-children-and-doing-unto-others-8139
Young, Robert John. “Could Biology Explain the Evolution of Religion.” The Conversation, May 28, 2014. https://theconversation.com/could-biology-explain-the-evolution-of-religion-24859