There are a few key things to bear in mind when writing for the web, this is a bit of a brain dump of ideas and I will add to it over time:
1. Keep it relevant. This is something which I struggle with myself, but if the content is not relevant to the topic on the page, a visitor will probably lose interest and move on.
2. Grab Their Interest. Get to the point right at the start, most web readers just skim the content, so if you make a bold statement or put the conclusion at the start, you stand a better chance of keeping them on the page.
3. One Idea In Each Paragraph. This relates back to keeping it relevant and helps to segment the article into manageable chunks.
4. Use Lists. Like this post, there is a brief header then a description for each point, so readers can skim or read the whole post easily.
5. Keep Sentences Short. Again, concise is the key here
6. Add links to relevant pages within the body of the Text. They will stand out as they should be a different colour and will make it clear to the reader that clicking it will move them to a new page with related content.
As I said, this list will evolve, but this is what springs to mind right now.