I use Ben Edelman's website to figure out which preferred seats are available. Then I find it easiest to e-mail delta through their website with my WP#, status and seat requests rather than waiting on hold to speak to somebody in India who has no idea what a Worldperks number is. Use an e-mail script along the lines of:

"I have an upcoming reservation, confirmation number ABC123, and would like to add my Northwest Worldperks number 987654321 to the reservation. As I have Skyteam Elite status and am therefore entitled to preferred seating, I would also like to reserve seat 12A--or if 12A is not available, seat 13A.

Please confirm the following:
1) My Northwest Worldperks number 987654321 has been added to the reservation.
2) My Skyteam Elite status is reflected in the reservation.
3) I have been moved to seat 12A or 13A."

It's important to be very clear, since the people answering the e-mails aren't always the best English language communicators. It once took my four e-mail exchanges to get them to confirm that they'd added both my and mrs. themicah's Worldperks numbers. "Please confirm that both our worldperks numbers have been added." "Yes, your worldperks number has been added." Etc.

The only thing is if you want an exit row, they'll sometimes refuse to assign it unless you call in, because you have to answer questions about your willingness/ability to help in the event of an emergency.