If you want to be copywriter, you've got to prove that you can think. You've got to be able to conceptualize, and the only way to show that you can do concepts is by doing ads, which brings us to another similarity between copywriter and art director portfolios: spec ads.
If you're just starting out and don't have any finished pieces in your book, spec ads are exactly what you need. Walk through stores and look for subjects for which making ads would be fun. Or, maybe you can invent a product which would be fun to work on. Or, maybe you're an expert on a certain subject. Do a campaign for it. You could re-do a bunch of "bad ads," too.
However you decide to put your portfolio together, make sure there are more ads in it than anything else.
There's another ability a copywriter must show in his portfolio: a copywriter must be a wordsmith. It's his responsibility to come up with the words that spell out an idea. Some of that ability comes from talent, but make no mistake about it, the craft of writing can be learned. It's not easy. It takes a long time. And the only teacher is experience. But it can be done.
Which brings us back to working on your portfolio.
When you arrive at a subject for which you want to do an ad, don't settle for the first headline you come up with. Do ten. Do twenty. Do a hundred. And when you've arrived at the one you really like, set it aside. Then write some more. Try to say the same thing in a different way. That kind of practice is the only way you'll ever learn to use your mind to produce advertising the way a concert pianist uses his instrument to produce beautiful music.
Follow the same advice if you decide to do taglines. Keep working your tags until you're crazy about them. Push yourself. You might be surprised how good you can be. This is not to say you shouldn't start showing your portfolio until you feel you're a polished professional. Quite the contrary: you'll never get to be a polished professional if you don't get a job, so start peddling your book as soon as you have ten or so ads and commercials that you like. But keep working on your book while you're looking for a job. Keep trying to replace those ten ads and commercials with ten better ads and commercials.
You've got to work just as hard on copy as you do on headlines and tags. So write it. Then write it again, and again, and again. If you want to be good, you've got to work hard.
Now let's talk about copywriters and pictures. Most advertising is a combination of words and pictures. But some writers have a hard time thinking visually when they're first starting out. This is okay: it doesn't doom would-be writers to permanent unemployment. I've known plenty of people who couldn't come up with a visual who came up with a junior copywriter's job. Then, as their careers developed, they learned to think visually.
Still, it's a fact of life that a copywriter who can think visually is worth more than one who can't, so thinking visually is something else young writers should work on.
Start out with obvious visuals that merely restate your headline. Then, slowly but surely, start working toward visuals that work with your headline to make a point, that make the meaning of your ads clear when the headline and visual are next to each other.
For example, let's say you were doing an ad for Jeeps. And let's say the product benefit you're trying to exploit is Jeep's 4-wheel drive. One way to do an ad would be to have a headline that says, "Jeep's 4-wheel drive makes it tough." With a visual that shows a Jeep climbing a hill.
It's a simple solution, but the visual does nothing besides restate the headline, and while this approach can't be completely condemned, it does leave us with a pretty boring ad. I don't think it would be more memorable than the other advertising next to which it would run. Personally, I'd be reluctant to hire someone with a portfolio full of ads like that.
Now suppose you looked at the some problem with an eye to making it say practically the same thing, only this time using a headline and visual which work together to make the sales point.
To wit: Headline: The shortest distance between two points is a Jeep.
To wit: Visual: Aerial shot of a Jeep leaving a paved road and going over hill and dale as a short cut.
Those are just two ways to work with visuals. There are lots of others, like testimonial ads: ads in which a real consumer spouts off about the benefits of a product; or first person statements, which showcase a product benefit through the eyes of an actor playing a consumer.
No one way is right all of the time. There are no rules determining which approach is most suitable for which advertising problem. That's why you should be a student of advertising and study every ad you can get your hands on. Analyze it. Figure out what makes it good or bad. How does the headline work with the visual? Could that headline have been better? Could there have been a better visual? If so, what?
You'll end up conducting a crude course in advertising for yourself. You'll learn that different advertising problems call for different creative approaches. You'll also end up learning a lot about how to use visuals, and about how to think visually.