Earlier this year I put together this summary of tips on writing for the web, at a client's request. I've found myself going back to it several times since then. Hope it helps!
Read this article by Jakob Nielsen (very well-known expert on web usability). It’s old, but still very relevant: “How Users Read on the Web.” In addition, here's my bullet-point summary of how to write for the web, combined from the Nielsen article as well as several others.