First things first, that's not necessarily a huge problem; lots of people see the red and think "the site is doomed". Depending on the topic, many sites get well under 15 questions per day and remain quite healthy and even graduate to full sites. It's quite probably the hardest metric to satisfy.
So, we're not doomed, we could just use more questions to keep getting more fresh content.
As for getting more questions, I really think they key things are growth and engaging new users. We've got a pretty good base of experienced Stack Exchange users who fulfill community moderation duties and plenty of people to answer most sorts of questions. However those experienced users don't tend to ask all that many questions, which is common on these sites.
So the focus is really on new users. We should share relevant questions and answers as much as reasonable and invite people to the site. Once new people are on the site, they should be given a warm welcome and informed of how to use the site. If a new user makes a mistake, help them. Tell them how to edit their post to get it in shape, or edit it yourself and tell them why. Invite them to chat (if they have enough reputation; 20 points as long as we're in Beta) and engage them.
We have a fair amount of one-off question users. That's not uncommon either, but remember each new poster has the potential to become a regular. Also remember the FAQ, Meta, privileged, the philosophy of this site...all this crap is pretty complicated. There's plenty of Just-In-Time help around the interface, but when there isn't UI help text, other users are the Just-In-Time help.