Good fitreps and a strong academic background. Economics wouldn't necessarily be something they'd be looking for but I think they look more at your overall academics instead of just your major.
I was a math major at the Academy and took Operations Analysis in Monterey, but I was in class with a girl who was an oceanography major as an undergrad. They give you an academic profile code based on gpa, technical course background, among other things. It's ranked and your academic background is evaluated and compared to what you're trying to major in at NPS to see if you'd fit.
All of it's explained in pretty good detail here
http://www.nps.edu/Academics/GeneralCatalog/Home.htm.
On another note, I went out there as an Ensign, but according to some of the LT's I worked with, I guess for pilots it used to be viewed negatively but now it's more neutral, with respect to promotions. Volunteering to take a hard-fill billet after NPS is supposed to help getting a slot there too I hear since it's a good pay-back tour.
*Edit* The Bupers instruction helps too
http://www.npc.navy.mil/CareerInfo/Education/