Copy writing is one of the most subjective thing involved with any marketing effort. Depending on each reader, you can get a different opinion every time. Every copy writer also has a certain style and if the reader does not fit the style, they won't care for it. It is important in copy writing to put someone in control, otherwise your marketing initiative can get way off track and over budget.
Good copy starts with understanding who your audience is and what it is you want them to walk away understanding or doing after reading your copy. You also need to make sure your copy matches your intent. Advertising, marketing, sales collateral, white papers, websites, emails, etc, all have different ways to write successful copy and you need to make sure that how you write it will connect with your audience and keep their interest and achieve the desired result.
The biggest thing to remember is with almost anything written, you only have the first paragraph (at the most) to determine if they will read any further. If it is an advertisement, it is the headline. Basically you have seven seconds to get their attention and interest or they move on - unless there is already an established interest in what they are reading.
Another important piece is graphics and design. The Internet has turned us into a visual society. The graphics and design of what you are saying need to reinforce the copy and message and go just as far as the copy itself into connecting with a particular audience.
Check out our creative brief in our tools section of this site. It provide a great structure to help you craft your message.