The first rule of copywriting is that you write for the audience. Period.
You write for humans first. You initiate a dialogue. You discuss your prose. You phrase your monologue. You provide information they seek. You provide information over and above what they seek. You create urgency where needed. You conflate ideas where needed. You introduce dogmas where required. You opine liberally. You thrive on needs. You sway ideologies where possible.
You understand that once they begin reading, your reader is your audience and you are the playmaster. Powerful copy can sell ice to eskimos, chipped from their own igloo. It can sell well-tailored suits to a hipster.
Contrary to popular belief. Much of your copy is about sales - Even when you're not explicitly selling a product. For instance, when you're writing for an information portal about a disease, you're selling authority. You're telling your readers to read it and subliminally getting them to believe you as an authoritative source for information on the disease. When you're writing for a tech blog about the new Apple iPhone, you're selling the idea of that blog being a news source on everything tech.
The confidence in your copy is the element much of your readers will devour. This is what sells. Has always sold.
Once that is done, the second point is readability. A copy that is technical, inundated with jargons is a turn off for most people. Write for the layman, no matter how knowledgeable you are on a topic or how in-depth your research is.
Finally, when all that is said and done - you come to optimization for search engines. This is where things get technical, this is also where you are more than likely to have your entire hard work thrown out. So be careful. Always, optimize at the end. Once your copy is complete.
Your optimization begins with the headline. I always create the copy first and then work on the headline. It ensures that the headline is based on my copy and my copy isn't limited by my headline. David Ogilvy, the father of advertising, had once very famously said that “On the average, five times as many people read the headline as read the body copy. When you have written your headline, you have spent eighty cents out of your dollar."
Once you're done with your headline, you move on to copy optimization. You take the main keyword and you then find derivatives. Yes, this is the writer's job - more often than not it is part of the research. If you make any claim of being an "SEO Copywriter" - keyword research is part of your job description. Finding derivatives, means getting long tail articles based on the seed keyword of the article.
You then merge your keyword with your copy, seamlessly. You don't enforce it.
You then merge your derivative keywords in your copy, seamlessly.
Finally, you wait a day and then proofread your work. This is where a lot of writers go wrong. Never try to proofread your work immediately upon completion. You are still in the "writing" mode and your brain will commit the same errors again and/or overlook discrepancies that you missed while writing. Once you wait for a day, you proofread your content with a fresh perspective and are more likely to catch errors that were previously made.
You look for forced paragraphs. You look for forced keywords.
Finally, take sometime to understand Latent Semantic Indexing - or LSI - a term that establishes values between words inside of a content to generate the meaning. This is partly mathematical. But it is an important aspect to understand.
Gone are the days when google ranked exact match keywords. For example go search for "best trucking company Louisiana" (without the quotes) in Google. On the results displayed, you'll see a few words highlighted. But you'll find that none of the sites that rank are using exact match keywords as is.
P.S - Typed on a phone at 6 in the morning. Please excuse typos and brevity.