Tricep extensions, tricep kickbacks, dips (using a chair or a bench).
I would add some other arm exercises (for shoulders and biceps) just to balance it out - arnolds, lateral flyes, W's, etc.
You can buy one of the P90X DVD's individually on eBay - look for "Shoulders and Triceps". There are also other exercises on it targeting triceps but I don't recall the names.
Correction - it's called "Chest, Shoulders and Triceps" - here is a
clicky.