First I have my patterns listed on Ravelry, Patternfish, and Craftsy. All at the same price point and each site seems to have their own specific market of knitters. What sells well on Ravelry doesn't really sell on the other two. Also, I know designers who sell on Etsy, Kollabora, and Amazon too.
Next, have a blog, facebook, twitter, and/or other social media sites where you can announce what you are working on, what you've published, and chat with your customers. I'm finding it takes a while to build up an audience on these social media sites. Some designers have held give-aways or hosted knit-a-longs on their sites to attract visitors. I haven't done this as of yet.
Lastly, I have purchased advertising through Ravelry. Ravelry is by far the most lucrative of the 3 sites I sell my patterns on and their advertising is very reasonable. I've only used the forum banner ads and think it has been good at getting my name seen by possible customers.
Bottom line ... it takes time (from what I've seen) to build up a customer base and pattern sales but it also takes my time. The more I'm on these social media sites the less time I have for designing and knitting, which are the parts of the business I love most. Start small ... nothing says you have to jump in to everything all at once!