I'm in off the waitlist!
Here's what worked for me. You can never really know for sure why you're waitlisted, but I assumed it was because of my weak quant background as a liberal arts major and lowish GMAT quant of 45. The very first thing I did was finish the mbamath.com certification that I had neglected to finish before applying and send in the transcript. The course was easy and took less than a week. I would only really recommend it if you're in a situation like I was. I also registered to retake the GMAT in February and registered for the HBX CORe program which I wouldn't have even started by the R2 deadline but I hoped would show how seriously I was taking things. A lot of people thought I was kind of crazy to bother with the GMAT again after getting a 740 the first time around but I couldn't shake the feeling that my 45 was holding me back after comparing it to admitted students.
I sent in my first update letter a couple days after officially registering my interest to stay on the waitlist back in January. This "letter" was 801 words and structured as an essay covering the following: how Northwestern was still my program of choice, the areas I suspected the adcom may have seen as my potential weak spots and how I was addressing them, and professional updates (including a promotion).
A month later I retook the GMAT and managed to bump my quant score up to a 48 for a total of 750. Sweet.
Based on advice from current students, I also got my assigned adcom officer's email address from the admissions office early and sent a short note saying hello and that NW was my top choice. From this point on I managed to find a reason to call the admissions office about once a week for the following five weeks for one reason or another, confirming they had received this or that document or asking for their opinion on shoring up a weak quant background. Mostly just noise to be honest. My adcom officer finally responded when I sent her another short note (at the admissions office's suggestion) regarding an upcoming professional deadline. The tone was exceptionally friendly and she encouraged me to keep sending updates. I sent another three emails over the next month leading up to the R2 deadline detailing some of the more interesting work I was doing for a major military training exercise. I never got (nor asked for) direct feedback on my application but her replies showed she was interested in what I had going on. When she called with the good news on the R2 deadline she mentioned that she had really appreciated the attention I had paid Northwestern throughout the process. I think this persistence really made all the difference.