Twitter, tumblr, facebook, etc. I'd recommend writers find blogs or websites the recommend content similar to what they're creating, and see if they can get added to rec lists.
Beyond this people often use Project Wonderful to pay for advertising. I'd wait until you have a good chunk of content before spending money, though. And if you're not artistically inclined, maybe hire an artist to help you make whatever kind of ad you go with.
Don't feel discouraged if growth is slow. Just do your best to put out quality content first and foremost. One of the worst things we see on the forum is people getting disheartened because they thought things would happen faster. Just do your best! Play the long game!