I hope I can give some external links here to point michelle to some resources. If not, please remove them, thanks
there is
The Ultimate Blogger Writing Guide | Copyblogger, a blog about writing for the web
and one of the old but gold
Writing for the Web (Jakob Nielsen's Alertbox)
jakob nielsen is an authority
you can install a Firefox plugin called senseo, it's very useful for on page optimization, and it recommends a keyword density of 5%-7%, I can't say if that's good or not, but the other tests it does are very helpful.
hope that helps