Do know your audience It's important to know your audience. If you write for a site that sells toys you'll use other words, colors, images etc. compared to a site for online banking. Write and design with your visitors in mind. Don't get tempted to write for yourself.
Don't use meaningless words Do you have a cool site with hot subjects? Or a hot site with cool subjects? On some hype-sensitive sites these kind of words might be useful but on most sites you'd better refrain from meaningless words.
Do write about subject Write about subject. Saying: This page is about breeding goldfish talks about page. Instead, start right away with subject. Breeding goldfish is a popular hobby....
Don't use jargon Avoid jargon. That goes for Internet jargon but also for jargon for any other subject. Only if your site is focused on a selective group of specialists jargon might make sense.
Do use short sentences Use short sentences. The World Wide Web is fast. Your visitors want to get your info in a snap. So read and reread your text. Then cut out as many unnecessary words as possible.
Don't write technical Don't write technical. Your visitors don't care how you created your site and that you prefer Perl over TCL/TK (or other way around). Instead write about your subject.
Do use correct spelling OK, this one will turn against me.... Use correct English or whatever language your site is written in. As a standard routine use a spelling checker but don't rely completely on it. Human proof reading is necessary. This can be difficult - especially if you're not native speaking English.
Don't show any page under construction Don't publish a page that's under construction. People will hate you if you do. If page isn't finished, it's not ready to be published. In a sense most pages are always under construction because they are updated (more or less) frequently.
Do use first screen Be sure to put important text on first part of your page, part that will show up first on a screen.