Lots of marathons and lots of prizes.
We've already been kicking around the idea of an answerthon, à la Travel.stackexchange.com. The idea to integrate a push for voting and flagging, possibly before the Answerthon, has also come up.
I like both of these ideas, and think that they would work very well on Ask Different. We've got a relatively low Percent Answered rate (8th from the bottom, at the time of this writing), and we also have less voting than many other sites. I think that a big push would not only help us with the backlog of questions we already have, but also help get a 'lots of voting' vibe in the air which would hopefully stick with people as they continue to use the site.
Answering and voting are both very important, but there's more to a successful, happy Q&A site than that. For Super User's 2nd birthday, they encouraged people to edit posts, edit tag wikis, and write for the blog (in addition to the things mentioned above). Why restrict people to votes, flags, and answers; I'd love to encourage people to do the smaller things that make this site great.
Oh, and Super User did more than encourage people to help the site: they rewarded them.
Prizes included swag and hardware/software.
Stack Exchange has offered to support something similar for us, if people are interested.
To see the exact categories of involvement and their rewards, take a look at the SU blog post. I really like their categories and think they would integrate very well with the marathons we've been thinking about.
If you've got ideas for categories and prizes, comment here!
We'll also need a panel of judges. If you're interested, comment here.