Go for the train! Definitely a great way to travel. When you arrive at AMS Schiphol, make your way to Schiphol Plaza and head over to the train tickets counter. For advice on travel to Dusseldorf, just ask at the NS International office.
Journey time from AMS airport to Amsterdam Centraal is 15-20 mins. Fare is NLG6.50 for second class or NLG10.00 first. Dutch Railways journey planner, English version can be found at:
http://www.ns.nl/reisplan2.asp
Journey from A'dam Centraal to Dusseldorf looks like about 2.5 hours if you catch a direct train.
Enjoy the ride and bring your camera!
Cheers,
FewMiles in the air, ManyMiles on NS trains!