I've learned so much about this in my own transition I barely know where to start.
It's generally known that reading on screen is slower and less comfortable than reading print. For me, this means that the job of keeping someone reading becomes much bigger. You've got give them a strong "hit" much more frequently. You can't afford to stretch out with long descriptive passages, elaborate sentences with lots of modifying clauses, or dry abstracted copy with no real meat but only corporate style market-speak.
I find that MS Word's grammar check is essential, especially the grade-level it provides at the end. I always aim for below 9th grade.
Here's a top-of-the head list:
Things to use:
1. Short sentences
2. Bullet points
3. VERY frequent subheads (hint - search engines like H tags)
4. Active voice
5. Precise nouns and verbs, instead of adjectives and adverbs
6. Anglo-Saxon roots rather than $10 Latinate words.
7. Redundancy - don't assume that a person reading paragraph 10 has already read paragraph 2. If there's an essential backward reference, spell it out.
Things to avoid:
1. Doubling both the verb and it's object. ("The results demonstrate and prove both the theory and its execution.")
2. Making the reader carry any information in their head from one part of the sentence to another ("Product ABC, from the days of its first release, and through the many years of testing and upgrade, has always been rated number one." -- THAT HURTS!)
I'm going to stop the list for now. One thing I would like to add - write some poetry. Most copy writers who have excelled LOVE words to the point of writing poems for pleasure.
Poetry naturally informs their marketing copy. The lines scan beautifully - when you read them aloud, they sound like natural speech, and even beautiful speech. They give pleasure to the reader. Rather than requiring effort and trying to convince, the copy attracts and entices. On the web, this is essential.
Copy that takes 1 minute to read may require 10 to 100 man-hours to create. But it can work for your website for many years, and repay that investment many times over (cliche! - avoid!). So don't fly solo! Use an editor - and optimally use a team.
How about it? Anyone else have some web copy tips to add?