Use layering.

• Layer smarter components on top of dumb ones
• Reuse library functions as much as possible. Internal reusability is a good predictor of external resuability.
• Layers add control and coordination that allow simple components to be used in complex ways.
• Layering results in ``black box'' reuse of full components rather than ``white box'' reuse-with-mangling of their parts. Properties (e.g., specs, tests) of the full components may be relied on without having to re-establish.