Pentel brush pens (JetPens) and Blick colored pencils work really well on cardboard boxes. Stabilo Colored Marking pencils (from Blick) are excellent for white highlights. The Kum two size sharpener (from Jet Pens) is also very handy.
The Pentel Brush pens have terrific line variations and work smoothly even on the cheapest cardboard.
Sometimes I'll do a quick sketch before hand. This is a Rhodia Dot pad (from Goulet Pens) and a Ohto Comfort 2mm lead holder from Jet Pens. I love these little pencils! An inexpensive leadholder with click advancement, a sharpener on the top and awesome colors too. I also have a green one.