Quality writing comes first. Care for your writing enough to be a rewriter, as opposed to dishing up the first version that comes out of your keyboard.
Presentation is very important: a friendly but professional-looking photo to go with an interesting bio, care taken with layout and formatting whenever publishing anything online, attention to proofreading (clean copy) and presenting information in a way that's easy for readers to digest.
Longevity is another key. Writers who want to be successful know they're in it for the long haul. Being consistent (whether on your own blog or on a site like HubPages) helps build a following. People know you'll always be there, representing your chosen genre or your own unique take on your topics.
Tasteful self-promotion is next, balanced with plenty of other types of communication so you don't sound like all you're doing is promoting yourself. Even though it can feel uncomfortable at first, some self-promotion is necessary if you want to get known. Becoming comfortable and sounding natural when you're marketing yourself and your writing products is better than a massive volume of promotion, which tends to get annoying or even boring.
Being genuinely interested in others is paramount. The online writing community is generous and supportive, and chock-full of talented writers/artists/artisans. It's not hard to spread the love. Being unselfish and willing to showcase others (because they deserve it, not as a tactic) can works wonders for a writer's online presence.