I suggest reviewing this thread:
Death-Knight-4.0.1-Tanking-Guide(first thread on the list)
There are tips and tricks on how to manage your runes better. Plus there are some add-on's that you might want to look at. As a DK tank since DK's were released it has taken some getting used to as it was a big change really, but once you get the hang of it it's not so bad.