When you are coding in a hurry, it is very tempting to write business logic in the first place that comes to mind, such as a button click handler. However, for all but the simplest systems, such a practice leads very quickly to a chaotic system whose business logic is scattered like the ash from an erupting volcano. Create a great domain model, though, and you will be able to nimbly align your software with your emerging business needs.
Every time I see an example of copy-and-paste programming, I'm like a bull who just spotted a waving red flag. "Don't Repeat Yourself" (DRY) is a principle every programmer should live by, for Turing's sake! Read on for a discussion of why copy-and-paste is egregious, and how to refactor an expansive set of conditional logic branches into a concise and elegant piece of code.