In general, to write persuasively for the web you should do these things:
- Be truthful, open, and considerate. Have your facts straight.
- Don't insult your reader accidentally by making them feel stupid - or on purpose by throwing around insults and generally being a jerk.
- Use legitimate appeals. Study up on rhetoric and make the kinds of appeals you want to make. Check your text to see if it's saying what you want in subtext.
- Use the register and semantics you want, considering your audience. This falls in with rhetoric appeal.
- Don't push too hard.