When you find you have to add a feature to a program, and the program's code is not structured in a convenient way to add the feature, first refactor… - Martin Fowler

Steve Mellor and I independently came up with a characterization of the three modes in which people use the UML: sketch, blueprint, and programming language. By far the most common of the three, at least to my biased eye, is UML as a sketch. In this usage, developers use the UML to help communicate some aspects of a system. As with blueprints, you can use sketches in a forward-engineering or reverse-engineering direction. Forward engineering draws a UML diagram before you write code, while reverse engineering builds a UML diagram from the existing code in order to help understand it.

Any fool can write code that a computer can understand. Good programmers write code that humans can understand.

Transparency is valuable, but while many things can be made transparent in distributed objects, performance isn't usually one of them.