I have been copywriting for a while offline and have just moved to the online environment. There are a lot of different factors to take into account as well. I have come across this post about SEO Copywriting hints on a blog called SeoUnique which have some truly great and innovative ways of writing copy.
Two examples which I consider to be great online copywriting advice are:
1. When writing around images or using images to backup your copy, give an accurate description of what the image is for or what it refers to.
2. Do not over use keywords in your individual page content. I have seen pages include a 3 keyword phrase more than 30 times on a single page and be placed 3 pages behind websites with 10-15 keywords on.
Anyway, I found the article very useful and I imagine other SEO copywriters would too, so enjoy!