Below are some good resources for writing for the web. Each title is a hot link, and I've tried to give you some idea of what each resource contains.
Microcontent: writing headlines, page titles, and email subject lines
Colorado State's Writing Guide for Email - Note: this link is to the "print-friendly" page for the information. When you click on the hot links on that page, it takes you to a non-print friendly page that's a bit cumbersome. All the information is on this page, so it's best not to use the hot links.
Tips on Writing Web Sites and Web page Optimization - Web Copy writing TipsThe Web Content Cafe's suggestions focus on commercial web sites. Some topics include:
How to Format Long Copy on Web Pages
Place Links in the Line of Sight
Too Many Choices Reduces Conversion Rates