WorldPerks University classes online will give 100 bonus miles instantly and a couple of the classes give 2,000 miles for your next roundtrip. Only do 1 a time as you do not get 4,000 miles if you complete 2 classes. Someone else will need to post a link for this because unfortunately the search on FT did not bring up the link.

You can also Supersize your next flight for $20 and get in your case 1,000 bonus miles. http://www.nwa.com/worldperks/supersize/

If you set up a Delta Frequent Flyer (SkyMiles) account and link it with your Northwest WorldPerks account you will get 500 miles posted to the Delta account which you can then transfer over to NW.

And here is the link for new account members and the 2,000 bonus miles after your first flight. http://www.nwa.com/features/wpenroll/