Modular designs are not only more easily recycled at the end of their life, but also more easily repaired and therefore last longer. Modular designs can also be more efficiently manufactured and shipped, reducing energy consumption at the beginning of the product's life.
Example: An example of this principle is the Aeron Chair by Herman Miller. As stated on Herman Miller's website, one of the design considerations for the Aeron was to make it "...sparing of natural resources, durable and repairable, designed for disassembly and recycling." As a result, Aeron chairs are not sent back to retailers for repair. Instead, if the chair breaks, replacement parts are ordered and the chairs are easily repaired on site.
Watch out: Making designs overly complicated in order to make them modular can do more harm than good. Adding extra fasteners, brackets, and materials opposes tips 4 and 14. Try to design for modularity that can be had for "free" using creative features on injection molded parts, or pieces of sheet metal that can accomplish multiple tasks.