Remember why your visitors came to the page and give them what they want.
Get to the point. The first two or three words may be all a visitor bothers to read in the Title, the meta title, the meta description, the headlines, paragraphs, photo alt text, etc.
Give the facts. Don’t say how great your business is- prove it.
Be brief but add more. Make every word count but at the same time the extra space means you have the opportunity to add relevant links, photos and other useful information.
Make it scannable. Website visitors will probably only read a fifth of the page. Give them short sentences, short paragraphs, sub headings, bullet points, highlighted words, photo captions and anything else that will slow their eye down for a moment.
Tell their story. A story that will take them where they want to go.
Tone of voice, house style, corporate brand- call it what you will but it must be consistent. Informal is usually what visitors like best.
Check it. Many visitors will judge you on spelling mistakes, links that don’t work, missing words, clichés, jargon and grammatical howlers like an apostrophe in the wrong place.
A longer version of this is available here 7 Tips For Writing Web Copy