Unholy remains my favorite tree, as it provides a good, solid amount of DPS provided with a lot of unique and fun utility and AE damage abilities. The build I will personally use to level my Death Knight will likely look a little something like this 16/0/55 Build, focusing on the Unholy skills first.
This build works best if you gather up a lot of enemies at once. Use Icy Touch and Plague Strike to Infect one mob, and Pestilence to spread the diseases to the other mobs. From there, you'll want to use Death and Decay (If you're DPSing in a group, make sure your tank has solid threat before you use it), and as runes permit, use Blood Boil and Scourge Strike. As soon as you have the Runic Power, you'll want to get Unholy Blight out for the extra damage If the battle lasts long enough, you'll probably need to reapply your diseases.
With this build, you'll want to keep your Ghoul and Bone Shield up as much as possible for the extra DPS. You can also use Corpse Explosion to explode your ghoul for massive damage (target the ghoul before using the ability) if you find yourself with an extra Unholy or Death Rune. Of course, if your health is looking low, you may decide to use Death Pact instead, once you have it. Night of the Dead will then assure that you'll be able to summon a new Ghoul almost immediately. Gargoyle is also a very massive helpful damage boost that you'll want to use as Runic Power permits.
If you want to be more a single target Unholy DPSer, or if you're fighting somewhere where casting AEs willy-nilly is inadvisable, you can simply cut back to your single-target DPS abilities, focusing on using Scourge Strike and Blood Strike for your damage once your diseases are properly applied.
The Unholy tree presents a small problem in using glyphs -- namely, there's so many good ones to use. Glyph of Bone Shield is essential, since it makes sure Bone Shield stays up longer, giving you more damage absorption and more DPS. When it comes to your second and eventually your third glyph, Glyph of Scourge Strike and Glyph of Ghoul will both be solid choices. If you have a hard time keeping your diseases up, or just don't want to deal with constant disease reapplication, you may want to pick up Glyph of Scourge Strike. If you're pretty confident in keeping your diseases up, though, Glyph of the Ghoul is a good choice to grab first.