If you are a member of NW's frequent flyer program you should be able to fly in the Premium class.
Nope... if you're Elite, you can reserve the seat at booking, if not, then you'll have to slide into it via the method I'll mention below.
NWA.com will tell you that the premium seats are reserved for WorldPerks Elite members. What they don't tell you is that those Elite members are upgraded into First automatically, beginning 5 days before the flight according to status. The seats vacated by upgraded passengers will be yours to choose from a few hours before the flight, first-come, first-serve.
With this in mind, your best idea would be to skip the online check-in idea in favor of an E-Ticket machine at the airport. When the machine gives you your seating assignment, instead of clicking "Proceed", select "Change Seat". Premium seats will be open at this time unless you get to the airport far too early, which will allow you to select exit row seats, bulkhead seats, or any location more desirable than 20E.
Create your own luck.