People aren’t reading online.
With that in mind, your goal should be to make your text easy to scan.
Use meaningful headings and subheadings
Use bulleted lists
Limit paragraphs to one idea
Use the inverted pyramid, putting the most important information at the beginning
Use fewer words
Use simple, clear language; readers hate marketese and gobbledygook
found on Writing, Clear and Simple
See also Jakob Nielsen’s Writing for the Web tips.
Update: I have added this to the graduate students category because if the students are writing on the net for future schools or employers to read, this is important stuff to know.