In many ways, good copywriting is good copywriting- it needs to be clear, compelling and informative. But when considering copy specifically for the web here are some tips:
Conduct some search term research before you begin writing or editing.
Whether or not you are interested in Search Engine Optimization or Search Engine Advertising, a valuable by-product of search term research is that you will know what terms your audience is using to identify your product or service. By using the language of your audience you will deliver your message more clearly and quickly.
Write for the audience - not your boss.
This is an extension of #1, but you need to fight the tendency to write copy that appeals to an industry insider, because that same copy is likely to confuse and turn off your target audience. Leave the terminology and acronyms in the conference room.
Look to your competitors websites for hints on language, organization and depth.
Chances are that your viewers are going to be looking at several websites in their time online and you need to be conscious of how your message plays in comparison. Are you more professional? More friendly? More or less informative?
If you can’t be clear while being clever, don’t be clever.
Too many headlines, captions and taglines fail miserably because they don’t clearly tell the viewer what they want to know - ‘what is this about?’
Use every opportunity to give the viewer information without having to read excessively.
This applies foremost to graphic design, but copywriters can help out by writing good headlines, good subheads, short captions, lists, and by using bolds and italics appropriately.
Watch the physical length and depth of your copy.
Sentences that are more than 12 words wide and paragraphs that are more than 12 lines deep are screaming not to be read. Good graphic design is about clustering. Create copy that works in manageable clusters.
Be conscious of spacing, type size and color.
It is already a struggle to get anyone to read commercial copy these days, don’t make it harder by displaying the text smaller or bigger or lighter or more spread out then it needs to be (unless you don’t want it read).
Strive to offer rich, relevant, compelling content to the user.
Commercial copywriting has always been a blend of fact and fiction, but the reason people don’t read is because they are weary of spin. But you can use this to your advantage if you can convince your audience that you are one of the few resources that gives them important, reliable information.