1. Clarity: Keep It Simple -- Aim for Instant Comprehension
If your prospect doesn't understand your copy or offer, if it confuses him, or if he doubts your credibility, he won't buy.
Therefore your first aim in any piece of copy is to explain your offer so that it can be understood by someone who's distracted -- no one reads advertising with anything more than mild interest.
Use short words. If you have the space, as in a direct mail letter, or an online sales page, tell a story. This will help to draw your prospect in and change his mind state to one of receptivity.
When you've completed your first draft, give your copy to a child of ten to read. If the child can understand it, you're golden.
2. Empathy: Understand Your Target Market
If you don't understand your target market, you can't sell to them.
Find out as much as you can about the audience for your product, and then put yourself in their shoes -- use your imagination. It helps to read magazines read by your target audience. You need to build empathy in any way you can.
The biggest mistake I see with beginning copywriters is that they don't spend enough time researching the audience, and their product.
A primary reason top copywriters make a great income is that they don't stint on the basics. They make their copy as clear as they can, they know the audience -- and they ensure that their offer is irresistible.
3. Reward: Your Offer Must Be Irresistible
Even if you write the most amazing copy in the world, if the offer isn't irresistible, people won't buy.
How do you create an irresistible offer? You must know the audience, and test.
A tip: discourage your copywriting client from competing on price in the offer -- instead, encourage him to add value.