The cornerstone of any successful web site is content. Without good, unique content, you might as well give up now. If you are fortunate enough to have good content for your site, whatever form that should take, you are off to a good start. Good content by itself, though, will not make you a millionaire any time soon. Without content, you have nothing to promote.
However amazing and unique your content is, it is no use unless people can (and do) find it. You might be very pleased that your latest article on "Accessible Web Design in Maseru, Lesotho" made it to the number one spot in Google when you search for the title, but that's not going to be very good for business if that's the only thing you rank well for. The news gets worse - while it might be easy to rank well for a phrase like the above, ranking well for more competitive terms may take a lot more work.
It is also important to remember that there are many strings to a successful marketing strategy, and search engine marketing is just one of those. That, in turn, has many different parts, most important of which are link building and copywriting - but do consider other avenues of promotion and marketing in addition.
Copywriting, when used in a Search Engine Optimisation (also known as "SEO"; meaning to make your pages rank highly in search results) sense, is the art of making your web pages seem both attractive to your users, and rank well for your chosen keyphrases. You can sometimes achieve a good rank using copywriting alone, however it is most effective as part of a more comprehensive strategy.
Choose and use your keywords wisely
Your keywords and keyphrases are those words and phrases that you wish to target. That is to say, you would like the copy you are writing to appear high up in the search engine results for users searching for those words and phrases. In order to make the most of these, it is usually a good idea to select the words and phrases you want to target before writing. Make a list of them, and keep it nearby, and mention them whenever appropriate.
I say "appropriate" because it is very easy to be over-enthusiastic and add every remotely related keyword to your copy. When writing your copy, remember that you are writing it to be read by users. A user faced with a page stuffed with keywords will be turned off. If your copy does not read well because you have used too many keywords, re-write it. It is far better to have a page just below the top place in the search results for a phrase that can entice people to buy your product (or sign up to your site, bookmark your page, or at the very least return to your site) than one that ranks at the top spot, but that puts off every potential user.
Your potential users may search for different keywords with identical, or close, meaning to the ones you think of first. Keeping a thesaurus handy can help you to find appropriate synonyms to sprinkly through your copy, thereby targetting those users as well. If your aim is to sell Jack Daniels, you might well be targeting the keywords "buy Jack Daniels" with your current copy - however, your potential users may be searching for "order Jack Daniels", or "purchase Jack Daniels", instead. If you can include variations on keywords in your text as well, you will be more comprehensively targeting your potential customers.
There are other tools available to find more information about the keywords you may want to target. Overture provides a free [url=http://inventory.overture.com/d/searchinventory/suggestion/]Search Term Suggestion Tool[/url] that can help when selecting a useful set of the keywords to target.
KISS - Keep It Simple, Stupid
There is little benefit to writing an article explaining why Jack Daniels is great if over half of it is filled with chemical symbols for what goes into it and explanations of the reactions that occur when you drink it. Over-complicating the subject will put the users off. The same applies to language. While you might feel that using the largest possible words will make you seem brighter and therefore better in the eyes of your users, it most likely will not - what it may well do is bore or confuse them.
Know your users
Write for your users, where possible. Try to imagine what your users want from your site, and the kind of language they are likely to want to find it in. If writing copy for a children's website, do not use large words, and don't be condescending. Do write simply and clearly, and explain things in an unambiguous, concise way where necessary.
Write for your marketplace too. If you are writing reviews of rap albums, for example, try to write in an appropriate style for the audience. That's not to say you should offer "props" to your "homies" every line. Rather, try to imagine yourself speaking to someone involved with that scene. Perhaps you have a cousin who's into rap music - how would you talk to him or her about the album? Would you use different language, or different sentence lengths? Probably - I know there would be a huge difference in the way I explained an Eminem track to my grandmother compared to how I explained it to my brother.
Put yourself in their shoes
Imagine you found the copy you are writing after a search for a certain keyword or phrase. What are you hoping to find from this article? Are you wondering whether you should go and see a certain movie or not? Planning your next holiday? Or are you just trying to find out the price of a new carburettor for your Jaguar? Try to create a list of the questions you might want the article to answer, and aim to answer all of them. The answer can be brief - but it should be there somewhere.
Users are not stupid. If you claim your product or service is "the best", they will want evidence of that. Claiming that your product or service is far superior to everything else available is likely to put them off buying at all, unless you can back that up with evidence. By toning down your copy, you may risk the loss of a couple of sales to impressionable souls who believe all they read. However, by not toning it down, you may lose far more sales to those who become suspicious of copy that hypes a product too strongly.