Keep in handy a pen drive containing your best write ups. Yes, you need to be selective; we all know the nonsense we write in the early days of our content writing career and even when we just don't feel like proof reading some write up, so skip such stuff. It's always advisable to show a wide variety of write-ups like blogs, articles, home page and on different topics. Also if the write-up is under your name it's even better for sake of authenticity. But as we know copywriting signifies that the identity of the actual writer is not disclosed so I'll suggest another way to prove your credentials. Post some articles on varied topics like technology, travel, tourism etc on article submission sites like Hubpages, Ezine, Buksia and so on. I did that and it paid off in a big way. Writers looking for a career in web journalism should have posted related articles on some online magazines or current affairs sites.