You can't really "encourage more questions", per se; not at least in any sustainable way. But getting more questions is a nice side effect of promoting healthy growth overall.
To make this site successful, you need a place where people are asking very interesting and challenging questions, not the same questions asked 100 times before on every other discussion site. The best way we've found to bring in more users is by publicizing interesting questions. Use those social links: Twitter, Facebook, etc.
Most of these sites start out with steady traffic going kind of horizontally for a while. Then, at some unpredictable point, POW the site hits a critical mass "tipping point" and the traffic starts climbing inexorably. Your traffic trends are pretty healthy, and there have been some promising signs of upward growth. Almost 52% of this site's traffic comes from questions people found in Google, so it looks like you are almost there!
To keep those users and get them asking good questions, the quality has to be there.
I tell you this because when people start talking about "getting more questions," the discussion inevitably goes in one or two directions: "seeding" the site with more questions, or lowering the bar on quality to keep more questions. Both are a losing formula.
Back when Stack Exchange was a for-subscription service, individuals tried to create these sites with little idea of how to draw an audience. When the questions did not magically appear, most panicked. Sites started soliciting and allowing lower-quality questions under the mistaken belief that "more questions at any cost" was better than nurturing fewer questions with the high quality Stack Exchange users have become accustomed to. The result was almost 3,000 sites shut down with a 99.6% failure rate.
I can't emphasize enough the importance of staying on top of the quality of your content. This site's quality remains high; nearly 100% of the questions get answered; you have a strong community of avid users… Those are your saving graces.
Stay on top of the quality. Promote interesting questions. The traffic will come. If you start allowing questions that aren't a step above, it will start to undermine what we believe makes these sites worthwhile in the first place.